A system of units of measurement, also known as a system of units or system of measurement, is a collection of

units of measurement A unit of measurement, or unit of measure, is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other qua ...

and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and

commerce Commerce is the organized Complex system, system of activities, functions, procedures and institutions that directly or indirectly contribute to the smooth, unhindered large-scale exchange (distribution through Financial transaction, transactiona ...

. Instances in use include the

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

or (the modern form of the

metric system The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...

), the British imperial system, and the

United States customary system United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that ...

History

In antiquity, ''systems of measurement'' were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in ''hands'' and ''knuckles''. The unifying characteristic is that there was some definition based on some standard. Eventually ''

cubit The cubit is an ancient unit of length based on the distance from the elbow to the tip of the middle finger. It was primarily associated with the Sumerians, Egyptians, and Israelites. The term ''cubit'' is found in the Bible regarding Noah ...

s'' and '' strides'' gave way to "customary units" to meet the needs of merchants and scientists. The preference for a more universal and consistent system only gradually spread with the growth of international trade and science. Changing a measurement system has costs in the near term, which often results in resistance to such a change. The substantial benefit of conversion to a more rational and internationally consistent system of measurement has been recognized and promoted by scientists, engineers, businesses and politicians, and has resulted in most of the world adopting a commonly agreed metric system. The French Revolution gave rise to the

, and this has spread around the world, replacing most customary units of measure. In most systems,

length Length is a measure of distance. In the International System of Quantities, length is a quantity with Dimension (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...

(distance),

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

, and

time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...

are ''base quantities''. Later, science developments showed that an electromagnetic quantity such as

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

or electric current could be added to extend the set of base quantities.

Gaussian units Gaussian units constitute a metric system of units of measurement. This system is the most common of the several electromagnetic unit systems based on the centimetre–gram–second system of units (CGS). It is also called the Gaussian unit syst ...

have only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such as power and

speed In kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. Intro ...

, are derived from the base quantities: for example, speed is distance per unit time. Historically, a wide range of units was used for the same type of quantity. In different contexts length was measured in

inch The inch (symbol: in or prime (symbol), ) is a Units of measurement, unit of length in the imperial units, British Imperial and the United States customary units, United States customary System of measurement, systems of measurement. It is eq ...

es,

feet The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up of ...

, yards,

fathom A fathom is a unit of length in the imperial and the U.S. customary systems equal to , used especially for measuring the depth of water. The fathom is neither an international standard (SI) unit, nor an internationally accepted non-SI unit. H ...

s, rods, chains,

furlong A furlong is a measure of distance in imperial units and United States customary units equal to one-eighth of a mile, equivalent to any of 660 foot (unit), feet, 220 yards, 40 rod (unit), rods, 10 chain (unit), chains, or a ...

mile The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a imperial unit, British imperial unit and United States customary unit of length; both are based on the older English unit of Unit of length, le ...

nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude at t ...

s, stadia, leagues, with conversion factors that were not based on power of ten. In the metric system and other recent systems, underlying relationships between quantities, as expressed by formulae of physics such as

Newton's laws of motion Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows: # A body re ...

, is used to select a small number of base quantities for which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten. For example, where the unit of length is the

metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

; a distance of 1 metre is 1,000 millimetres, or 0.001 kilometres.

Current practice

Metrication is complete or nearly complete in most countries. However,

US customary units United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that ...

remain heavily used in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and to some degree in

Liberia Liberia, officially the Republic of Liberia, is a country on the West African coast. It is bordered by Sierra Leone to Liberia–Sierra Leone border, its northwest, Guinea to Guinea–Liberia border, its north, Ivory Coast to Ivory Coast–Lib ...

. Traditional Burmese units of measurement are used in

Burma Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and ha ...

, with partial transition to the metric system. U.S. units are used in limited contexts in Canada due to the large volume of trade with the U.S. There is also considerable use of imperial weights and measures, despite ''de jure'' Canadian conversion to metric. A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom whose road signage legislation, for instance, only allows distance signs displaying

imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thr ...

(miles or yards) or Hong Kong.HK Weights and Measures Ordinance
/ref> In the United States, metric units are virtually always used in science, frequently in the military, and partially in industry. U.S. customary units are primarily used in U.S. households. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than

grains A grain is a small, hard, dry fruit ( caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and le ...

, are used for medications. Some other non- SI units are still in international use, such as

s and

knots A knot is a fastening in rope or interwoven lines. Knot or knots may also refer to: Other common meanings * Knot (unit), of speed * Knot (wood), a timber imperfection Arts, entertainment, and media Films * ''Knots'' (film), a 2004 film * ''Kn ...

in aviation and shipping, and

for aircraft altitude.

Metric system

Metric system The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...

s of units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries. Multiples and submultiples of metric units are related by powers of ten and their names are formed with

prefixes A prefix is an affix which is placed before the stem of a word. Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the word to which it is affixed. Prefixes, like other affixes, can b ...

. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units. In the early metric system there were two base units, the

for length and the

gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram. Originally defined in 1795 as "the absolute Mass versus weight, weight of a volume ...

for mass. The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units. Mesures usuelles ( French for ''customary measures'') were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was used in France from 1812 to 1839. A number of variations on the metric system have been in use. These include gravitational systems, the centimetre–gram–second systems (cgs) useful in science, the metre–tonne–second system (mts) once used in the USSR and the

metre–kilogram–second system The metre, kilogram, second system of units, also known more briefly as MKS units or the MKS system, is a physical system of measurement based on the metre, kilogram, and second (MKS) as base units. Distances are described in terms of metres, mass ...

(mks). In some engineering fields, like

computer-aided design Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...

, millimetre–gram–second (mmgs) is also used. The current international standard for the metric system is the

( or SI). It is a system in which all units can be expressed in terms of seven units. The units that serve as the

SI base unit The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which al ...

s are the

kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...

kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

, mole, and

candela The candela (symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radi ...

British imperial and US customary units

Both British imperial units and

derive from earlier

English unit English units were the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at ...

s. Imperial units were mostly used in the former

British Empire The British Empire comprised the dominions, Crown colony, colonies, protectorates, League of Nations mandate, mandates, and other Dependent territory, territories ruled or administered by the United Kingdom and its predecessor states. It bega ...

and the

British Commonwealth The Commonwealth of Nations, often referred to as the British Commonwealth or simply the Commonwealth, is an international association of 56 member states, the vast majority of which are former territories of the British Empire The B ...

, but in all these countries they have been largely supplanted by the metric system. They are still used for some applications in the United Kingdom but have been mostly replaced by the metric system in

commercial Commercial may refer to: * (adjective for) commerce, a system of voluntary exchange of products and services ** (adjective for) trade, the trading of something of economic value such as goods, services, information or money * a dose of advertising ...

scientific Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

, and industrial applications. US customary units, however, are still the main system of measurement in the

. While some steps towards

metrication Metrication or metrification is the act or process of converting to the metric system of measurement. All over the world, countries have transitioned from local and traditional Unit of measurement, units of measurement to the metric system. This ...

have been made (mainly in the late 1960s and early 1970s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development. While British imperial and US customary systems are closely related, there are a number of differences between them. Units of length and area (the

foot The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...

yard The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...

, etc.) have been identical since the adoption of the International Yard and Pound Agreement; however, the US and, formerly, India retained older definitions for surveying purposes. This gave rise to the US

survey foot The foot (standard symbol: ft) is a unit of length in the British imperial and United States customary systems of measurement. The prime symbol, , is commonly used to represent the foot. In both customary and imperial units, one foot compri ...

, for instance. The

avoirdupois Avoirdupois (; abbreviated avdp.) is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959. In 1959, by international agreement, the definitions of the p ...

units of mass and weight differ for units larger than a pound (lb). The British imperial system uses a stone of 14 lb, a long hundredweight of 112 lb and a

long ton The long ton, also known as the imperial ton, displacement ton,Dictionary.com - ''"a unit for measuring the displacement of a vessel, equal to a long ton of 2240 pounds (about 1016 kg) or 35 cu. ft. (1 cu. m) of seawater."'' or British ton, is a ...

of 2,240 lb. The

stone In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...

is not a measurement of weight used in the US. The US customary system uses the short hundredweight of 100 lb and

short ton The short ton (abbreviation: tn or st), also known as the US ton, is a measurement unit equal to . It is commonly used in the United States, where it is known simply as a ton; however, the term is ambiguous, the single word "ton" being variously ...

of 2,000 lb. Where these systems most notably differ is in their units of volume. An imperial fluid ounce of 28.4130625 ml is 3.924% smaller than the US

fluid ounce A fluid ounce (abbreviated fl oz, fl. oz. or oz. fl., old forms ℥, fl ℥, f℥, ƒ ℥) is a unit of volume (also called ''capacity'') typically used for measuring liquids. The British Imperial, the United States customary, and the United S ...

(fl oz) of 29.5735295625

millilitre The litre ( Commonwealth spelling) or liter ( American spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metres (m3). A cu ...

s (ml). However, as there are 16 US fl oz to a US

pint The pint (, ; symbol pt, sometimes abbreviated as ''p'') is a unit of volume or capacity in both the imperial and United States customary measurement systems. In both of those systems, it is one-eighth of a gallon. The British imperial pint ...

and 20 imp fl oz to an imperial pint, the imperial pint is 20.095% larger than a US pint, and the same is true for

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

quart The quart (symbol: qt) is a unit of volume equal to a quarter of a gallon. Three kinds of quarts are currently used: the liquid quart and dry quart of the US customary system and the of the British imperial system. All are roughly equal ...

s, and

gallon The gallon is a unit of volume in British imperial units and United States customary units. The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia ...

s: six US gallons (22.712470704 L) is only 0.08% less than five imperial gallons (22.73045 L). The

system served as the general system of mass and weight. In addition to this, there are the

troy Troy (/; ; ) or Ilion (; ) was an ancient city located in present-day Hisarlik, Turkey. It is best known as the setting for the Greek mythology, Greek myth of the Trojan War. The archaeological site is open to the public as a tourist destina ...

and the

apothecaries' system The apothecaries' system, or apothecaries' weights and measures, is a historical system of mass and volume units that were used by physicians and apothecaries for medical prescriptions and also sometimes by scientists."Medicinal-Gewicht, Apotheke ...

s. Troy weight was customarily used for

precious metal Precious metals are rare, naturally occurring metallic chemical elements of high Value (economics), economic value. Precious metals, particularly the noble metals, are more corrosion resistant and less reactivity (chemistry), chemically reac ...

black powder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...

, and

gemstone A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...

s. The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system. The apothecaries' system was traditionally used in

pharmacology Pharmacology is the science of drugs and medications, including a substance's origin, composition, pharmacokinetics, pharmacodynamics, therapeutic use, and toxicology. More specifically, it is the study of the interactions that occur betwee ...

, but has now been replaced by the metric system; it shared the same pound and ounce as the troy system but with different further subdivisions.

Natural units

Natural units In physics, natural unit systems are measurement systems for which selected physical constants have been set to 1 through nondimensionalization of physical units. For example, the speed of light may be set to 1, and it may then be omitted, equa ...

are

defined in terms of universal

physical constants A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally. It is distinct from a mathematical constant, which has a ...

in such a manner that selected physical constants take on the numerical value of one when expressed in terms of those units. Natural units are so named because their definition relies on only properties of

nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

and not on any human construct. Varying systems of natural units are possible, depending on the choice of constants used. Some examples are as follows: *

Geometrized unit system A geometrized unit system or geometrodynamic unit system is a system of natural units in which the base physical units are chosen so that the speed of light in vacuum, ''c'', and the gravitational constant, ''G'', are set equal to unity. : c = 1 \ ...

s are useful in

relativistic physics In physics, relativistic mechanics refers to mechanics compatible with special relativity (SR) and general relativity (GR). It provides a non- quantum mechanical description of a system of particles, or of a fluid, in cases where the velocities o ...

. In these systems,

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

and the

gravitational constant The gravitational constant is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...

are among the constants chosen. *

Planck units In particle physics and physical cosmology, Planck units are a system of units of measurement defined exclusively in terms of four universal physical constants: ''Speed of light, c'', ''Gravitational constant, G'', ''Reduced Planck constant, ħ ...

is system of geometrized units in which the

reduced Planck constant The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...

is included in the list of defining constants. It is based on only properties of

free space A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

rather than of any object or particle. *

Stoney units In physics, the Stoney units form a system of units named after the Irish physicist George Johnstone Stoney, who first proposed them in 1874 (but published only in 1881). They are the earliest example of natural units, i.e., a coherent set of unit ...

is a system of geometrized units in which the

Coulomb constant Coulomb's inverse-square law, or simply Coulomb's law, is an experimental scientific law, law of physics that calculates the amount of force (physics), force between two electric charge, electrically charged particles at rest. This electric for ...

and the

elementary charge The elementary charge, usually denoted by , is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 ''e'') or, equivalently, the magnitude of the negative electric charge carried by a single electron, ...

are included. *

Atomic units The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested ...

are a system of units used in

atomic physics Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned wit ...

, particularly for describing the properties of

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s. The atomic units have been chosen to use several constants relating to the electron: the

electron mass In particle physics, the electron mass (symbol: ) is the mass of a stationary electron, also known as the invariant mass of the electron. It is one of the fundamental constants of physics. It has a value of about or about , which has an energy ...

, the

and the

. The unit of

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

in this system is the total energy of the

in the Bohr atom and called the

Hartree energy The hartree (symbol: ''E''h), also known as the Hartree energy, is the unit of energy in the atomic units system, named after the British physicist Douglas Hartree. Its CODATA recommended value is = The hartree is approximately the negative ...

. The unit of length is the

Bohr radius The Bohr radius () is a physical constant, approximately equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an at ...

Non-standard units

Non-standard measurement units also found in books, newspapers etc., include:

Area

* The

American football field The rectangular field of play used for American football games measures long between the goal lines, and (53.3 yards) wide. The field may be made of grass or artificial turf. In addition, there are two end zones on each end of the field, ext ...

, which has a playing area long by wide. This is often used by the American public media for the sizes of large buildings or parks. It is used both as a unit of length (, the length of the playing field excluding goal areas) and as a unit of area (), about . * British media also frequently uses the

football pitch A football pitch or soccer field is the playing surface for the game of association football. Its dimensions and markings are defined by Law 1 of the Laws of the Game (association football), Laws of the Game, "The Field of Play". The pitch is ty ...

for equivalent purposes, although

soccer Association football, more commonly known as football or soccer, is a team sport played between two teams of 11 Football player, players who almost exclusively use their feet to propel a Ball (association football), ball around a rectangular f ...

pitches are not of a fixed size, but instead can vary within defined limits ( long, and wide, giving an area of ). However the

UEFA Champions League The UEFA Champions League (UCL) is an annual club association football competition organised by the UEFA, Union of European Football Associations (UEFA) that is contested by List of top-division football clubs in UEFA countries, top-divisio ...

field must be exactly giving an area of or . For example, " HSS vessels are aluminium catamarans about the size of a football pitch." * Larger areas are also expressed as a multiple of the areas of states and countries understood to be familiar to the reader.

Energy

* A ton of

TNT equivalent TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. A ton of TNT equivalent is a unit of energy defined by convention to be (). It is the approximate energy released in the de ...

, and its multiples the kiloton, the megaton, and the gigaton. Often used in stating the power of very energetic events such as

explosion An explosion is a rapid expansion in volume of a given amount of matter associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Explosions may also be generated ...

s and

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

events and

earthquakes An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they c ...

and

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

impacts. A

of TNT as a unit of

has been defined as 1000 thermochemical calories (). * The atom bomb dropped on Hiroshima. Its energy yield is often used in the public media and popular

books A book is a structured presentation of recorded information, primarily verbal and graphical, through a medium. Originally physical, electronic books and audiobooks are now existent. Physical books are objects that contain printed material, mo ...

as a unit of energy. (Its yield was roughly 13 kilotons, or 60 TJ.) * One stick of

dynamite Dynamite is an explosive made of nitroglycerin, sorbents (such as powdered shells or clay), and Stabilizer (chemistry), stabilizers. It was invented by the Swedish people, Swedish chemist and engineer Alfred Nobel in Geesthacht, Northern German ...

Units of currency

A unit of measurement that applies to

money Money is any item or verifiable record that is generally accepted as payment for goods and services and repayment of debts, such as taxes, in a particular country or socio-economic context. The primary functions which distinguish money are: m ...

is called a

unit of account In economics, unit of account is one of the functions of money. A unit of account is a standard numerical monetary unit of measurement of the market value of goods, services, and other transactions. Also known as a "measure" or "standard" of ...

in economics and unit of measure in accounting. This is normally a

currency A currency is a standardization of money in any form, in use or circulation as a medium of exchange, for example banknotes and coins. A more general definition is that a currency is a ''system of money'' in common use within a specific envi ...

issued by a

country A country is a distinct part of the world, such as a state, nation, or other political entity. When referring to a specific polity, the term "country" may refer to a sovereign state, state with limited recognition, constituent country, ...

or a fraction thereof; for instance, the

US dollar The United States dollar (symbol: $; currency code: USD) is the official currency of the United States and several other countries. The Coinage Act of 1792 introduced the U.S. dollar at par with the Spanish silver dollar, divided it int ...

and US cent ( of a dollar), or the

euro The euro (currency symbol, symbol: euro sign, €; ISO 4217, currency code: EUR) is the official currency of 20 of the Member state of the European Union, member states of the European Union. This group of states is officially known as the ...

and euro cent.

ISO 4217 ISO 4217 is a standard published by the International Organization for Standardization (ISO) that defines alpha codes and numeric codes for the representation of currencies and provides information about the relationships between individ ...

is the international standard describing three letter codes (also known as the currency code) to define the names of currencies established by the International Organization for Standardization (ISO).

Historical systems of measurement

Throughout history, many official systems of measurement have been used. While no longer in official use, some of these customary systems are occasionally used in day-to-day life, for instance in

cooking Cooking, also known as cookery or professionally as the culinary arts, is the art, science and craft of using heat to make food more palatable, digestible, nutritious, or Food safety, safe. Cooking techniques and ingredients vary widely, from ...

Africa

* Algerian *

Egyptian ''Egyptian'' describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of year ...

Ethiopian Ethiopians are the native inhabitants of Ethiopia, as well as the global diaspora of Ethiopia. Ethiopians constitute several component ethnic groups, many of which are closely related to ethnic groups in neighboring Eritrea and other parts of ...

* Eritrean * Guinean * Libyan * Malagasy * Mauritian * Moroccan * Seychellois * Somalian * Tunisian * South African * Tanzanian

Asia

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

M. Ismail Marcinkowski, ''Measures and Weights in the Islamic World. An English Translation of Professor Walther Hinz's Handbook "Islamische Maße und Gewichte"'', with a foreword by Professor Bosworth, F.B.A. Kuala Lumpur, ISTAC, 2002, . This work is an annotated translation of a work in German by the late German orientalist Walther Hinz, published in the ''Handbuch der Orientalistik'', erste Abteilung, Ergänzungsband I, Heft 1, Leiden, The Netherlands: E. J. Brill, 1970. *

Afghan Afghan or Afgan may refer to: Related to Afghanistan *Afghans, historically refers to the Pashtun people. It is both an ethnicity and nationality. Ethnicity wise, it refers to the Pashtuns. In modern terms, it means both the citizens of Afghanist ...

* Cambodian * Chinese **

Hong Kong Hong Kong)., Legally Hong Kong, China in international treaties and organizations. is a special administrative region of China. With 7.5 million residents in a territory, Hong Kong is the fourth most densely populated region in the wor ...

Hebrew Hebrew (; ''ʿÎbrit'') is a Northwest Semitic languages, Northwest Semitic language within the Afroasiatic languages, Afroasiatic language family. A regional dialect of the Canaanite languages, it was natively spoken by the Israelites and ...

( Biblical and Talmudic) * Indian * Indonesian * Japanese * Korean *

Omani Oman, officially the Sultanate of Oman, is a country located on the southeastern coast of the Arabian Peninsula in West Asia and the Middle East. It shares land borders with Saudi Arabia, the United Arab Emirates, and Yemen. Oman’s coastline ...

* History of measurement systems in Pakistan, Pakistani * Philippine units of measurement, Philippine * Ancient Mesopotamian weights and measures, Mesopotamian * Ancient Persian weights and measures, Persian * Singaporean units of measurement, Singaporean * Sri Lankan units of measurement, Sri Lankan * Syrian units of measurement, Syrian * Taiwanese units of measurement, Taiwanese * Tamil units of measurement, Tamil * Thai units of measurement, Thai * Vietnamese units of measurement, Vietnamese * Nepalese units of measurement, Nepalese Still in use: * Myanmar units of measurement, Myanmar

Europe

* Ancient Greek units of measurement, Ancient Greek * Belgian units of measurement, Belgian * Byzantine units of measurement, Byzantine * Czech units of measurement, Czech * Cypriot units of measurement, Cypriot * Danish unit, Danish * Dutch unit, Dutch * English unit, English * Estonian units of measurement, Estonian * Finnish unit, Finnish * French units of measurement, French (now) * Units of measurement in France before the French Revolution, French (to 1795) * German unit, German * Greek units of measurement, Greek * Hungarian units of measurement, Hungary * Icelandic units of measurement, Icelandic *Old Irish units of measurement, Irish * Italian units of measurement, Italian * Latvian units of measurement, Latvian * Luxembourgian units of measurement, Luxembourgian * Maltese units of measurement, Maltese * Norwegian units of measure, Norwegian * Old Polish units of measurement, Polish * Portuguese customary units, Portuguese * Ancient Roman weights and measures, Roman * Romanian units of measurement, Romanian * Obsolete Russian weights and measures, Russian * Obsolete Scottish units of measurement, Scottish * Serbian units of measurement, Serbian * Slovak units of measurement, Slovak * Spanish customary units, Spanish * Swedish unit, Swedish * Swiss units of measurement, Switzerland * Ottoman units of measurement, Turkish * Obsolete Tatar weights and measures, Tatar * Welsh units of measurement, Welsh

North America

* Costa Rican units of measurement, Costa Rican * Cuban units of measurement, Cuban * Haitian units of measurement, Haitian * Honduran units of measurement, Honduran * Mexican units of measurement, Mexico * Nicaraguan unit of measurement, Nicaraguan *Puerto Rican units of measurement, Puerto Rican

South America

*Argentine units of measurement, Argentine *Bolivian units of measurement, Bolivian *Brazilian units of measurement, Brazilian *Chilean units of measurement, Chilean *Colombian units of measurement, Colombian *Paraguayan units of measurement, Paraguayan *Peruvian units of measurement, Peruvian *Uruguayan units of measurement, Uruguayan *Venezuelan units of measurement, Venezuelan

Ancient

*Ancient Arabic units of measurement, Arabic * Biblical and Talmudic *Ancient Egyptian units of measurement, Egyptian *Ancient Greek units of measurement, Greek *Hindu units of time, Hindu (time) *History of measurement systems in India, Indian *Ancient Mesopotamian units of measurement, Mesopotamian *Persian units of measurement, Persian *Ancient Roman units of measurement, Roman

Notes and references

Bibliography

* Tavernor, Robert (2007), ''Smoot's Ear: The Measure of Humanity'',

External links

CLDR – Unicode localization of currency, date, time, numbersA Dictionary of Units of Measurement
*Reasonover's Land Measure

*[https://web.archive.org/web/20060226162744/http://aurora.regenstrief.org/~schadow/units/UCUM/ucum.html The Unified Code for Units of Measure] {{SI units Systems of units, * Measurement

History

Current practice

Metric system

British imperial and US customary units

Natural units

Non-standard units

Area

Energy

Units of currency

Historical systems of measurement

Africa

Asia

Europe

North America

South America

Ancient

See also

Notes and references

Bibliography

External links