Each prefix name has a symbol that is used in combination with the symbols for units of measure. For example, the symbol for kilo- is k, and is used to produce km, kg, and kW, which are the SI symbols for kilometre, kilogram, and kilowatt, respectively. Except for the early prefixes of kilo-, hecto-, and deca-, the symbols for the multiplicative prefixes are uppercase letters, and those for the fractional prefixes are lowercase letters.[2] There is a Unicode symbol for micro µ for use if the Greek letter μ is unavailable.[Note 1] When both are unavailable, the visually similar lowercase Latin letter u is commonly used instead. SI unit symbols are never italicised.
Prefixes corresponding to an integer power of one thousand are generally preferred. Hence 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes deci-, and centi-, and less frequently hecto- and deca-, are commonly used for everyday purposes, and the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion".[3]
Prefixes may not be used in combination. This also applies to mass, for which the SI base unit (kilogram) already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg).
In the arithmetic of measurements having units, the units are treated as multiplicative factors to values. If they have prefixes, all but one of the prefixes must be expanded to their numeric multiplier, except when combining values with identical units. Hence:
- 5 mV × 5 mA = 5×10−3 V × 5×10−3 A = 25×10−6 V⋅A = 25 μW.
- 5.00 mV + 10 μV = 5.00 mV + 0.01 mV = 5.01 mV.
When powers of units occur, for example, squared or cubed, the multiplicative prefix must be considered part of the unit, and thus included in the exponentiation:
- Examples
- 5 cm = 5×10−2 m = 5 × 0.01 m = 0.05 m.
- 9 km2 = 9 × (103 m)2 = 9 × (103)2 × m2 = 9×106 m2 = 9 × 1000000 m2 = 9000000 m2.
- 3 MW = 3×106 W = 3 × 1000000 W = 3000000 W.
Application to units of measurement
The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidynes and milligauss). Metric prefixes may also be used with non-metric units.
The choice of prefixes with a given unit is usually dictated by convenience of use. Unit prefixes for amounts that are much larger or smaller than those actually encountered are seldom used.
Metric units
Mass
The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. Megagram and teragram are occasionally used to disambiguate the metric tonne from other units with the name "ton".[citation needed]
The kilogram is the only base unit of the International System of Units that includes a metric prefix.
Volume
The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, are often measured in hectolitres (each 100 litres in size).
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific purposes, the cubic metre is usually used.
Length
The kilometre, metre, centimetre, millimetre, and smaller are common. (However, the decimetre is rarely used.) The micrometre is often still referred to by the older non-SI term micron. In some fields, such as chemistry, the ångström (equal to 0.1 nm) was historically used instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical units, light years, and parsecs; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.
Time
Prefixes for the SI standard unit second are most commonly encountered for quantities less than one second. For larger quantities, the system of minutes (60 seconds), hours (60 minutes) and days (24 hours) is accepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardized to 86400 seconds so as not to create issues with the irregular leap second.
Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy, the Julian year or annum is a standardized variant of the year, equal to exactly 31557600 SI seconds (365 days, 6 hours). The unit is so named because it was the average length of a year in the Julian calendar. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum or gigaannum.
Angle
The SI unit of angle is the radian, but degrees, minutes, and seconds see some scientific use.
Temperature
Official policy also varies from common practice for the degree Celsius (°C). NIST states:[3]
Prefixes may not be used in combination. This also applies to mass, for which the SI base unit (kilogram) already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg).
In the arithmetic of measurements having units, the units are treated as multiplicative factors to values. If they have prefixes, all but one of the prefixes must be expanded to their numeric multiplier, except when combining values with identical units. Hence:
When powers of units occur, for example, squared or cubed, the multiplicative prefix must be considered part of the unit, and thus included in the exponentiation:
- 1 km2 means one square kilometre, or the area of a square of 1000 m by 1000 m and not 1000 square metres.
- 2 Mm3 means two cubic megametres, or the volume of two cubes of 1000000 m by 1000000 m by 1Metric units
Mass
The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. Megagram and teragram are occasionally used to disambiguate the metric tonne from other units with the name "ton".[citation needed]
The kilogram is the only base unit of the International System of Units that includes a metric prefix.
Volume
The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, are often measured in hectolitres (each 100 litres in size).
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific purposes, the cubic metre is usually used.
Length
The kilometre, metre, centimetre, millimetre, and smaller are common. (However, the decimetre is rarely used.) The micrometre is often still referred to by the older non-SI term micron. In some fields, such as chemistry, the ångström (equal to 0.1 nm) was historically used instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical units, light years The choice of prefixes with a given unit is usually dictated by convenience of use. Unit prefixes for amounts that are much larger or smaller than those actually encountered are seldom used.
The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. Megagram and teragram are occasionally used to disambiguate the metric tonne from other units with the name "ton".[citation needed]
The kilogram is the only base unit of the International System of Units that includes a metric prefix.
Volume
The litre (equal to a cubic decimetre), millilitre (equal to a cubic The kilogram is the only base unit of the International System of Units that includes a metric prefix.
The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, are often measured in hectolitres (each 100 litres in size).
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific purposes, the cubic metre is usually used.
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