The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in²; abbreviation: psi) is a unit of

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

or of stress based on

avoirdupois The avoirdupois system (; 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 defini ...

units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 Pa. Pounds per square inch absolute (psia) is used to make it clear that the pressure is relative to a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...

rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around , this will be added to any pressure reading made in air at

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardis ...

. The converse is pounds per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psi) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential (psid).

Multiples

The kilopound per square inch (ksi) is a scaled unit derived from psi, equivalent to a thousand psi (1000 lbf/in²). ksi are not widely used for gas pressures. They are mostly used in materials science, where the

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials ...

of a material is measured as a large number of psi. The conversion in SI units is 1 ksi = 6.895 MPa, or 1 MPa = 0.145 ksi. The megapound per square inch (Mpsi) is another multiple equal to a million psi. It is used in

mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objec ...

for the elastic modulus of materials, especially for metals. The conversion in SI units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.

Magnitude

Inch of water Inches of water is a non- SI unit for pressure. It is also given as inches of water gauge (iwg or in.w.g.), inches water column (inch wc, in. WC, " wc, etc. or just wc or WC), inAq, Aq, or inHO. The units are conventionally used for measurement o ...

: 0.036 psid *

Blood pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure ...

– clinically normal human blood pressure (120/80

mmHg A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg. Although not an ...

): 2.32 psig/1.55 psig *

Natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon d ...

residential piped in for consumer appliance; 4–6 psig. * Boost pressure provided by an automotive turbocharger (common): 6–15 psig * NFL football: 12.5–13.5 psig * Atmospheric pressure at sea level (standard): 14.7 psia * Automobile tire overpressure (common): 32 psig * Bicycle tire overpressure (common): 65 psig * Workshop or garage air tools: 90 psig * Air brake (rail) or air brake (road vehicle) reservoir overpressure (common): 90–120 psig * Road racing bicycle tire overpressure: 120 psig * Steam locomotive fire tube boiler (UK, 20th century): 150–280 psig * Union Pacific Big Boy steam locomotive boiler: 300 psig *

US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

steam boiler pressure: 800 psi *

pipelines: 800–1000 psig * Full SCBA (self-contained breathing apparatus) for

IDLH The term immediately dangerous to life or health (IDLH) is defined by the US National Institute for Occupational Safety and Health (NIOSH) as exposure to airborne contaminants that is "likely to cause death or immediate or delayed permanent advers ...

(non-fire) atmospheres: 2216 psig *

Nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

primary loop: 2300 psi * Full

SCUBA Scuba may refer to: * Scuba diving ** Scuba set, the equipment used for scuba (Self-Contained Underwater Breathing Apparatus) diving * Scuba, an in-memory database developed by Facebook * Submillimetre Common-User Bolometer Array, either of two in ...

(self-contained underwater breathing apparatus) tank overpressure (common): 3000 psig * Full SCBA (self-contained breathing apparatus) for interior firefighting operations: 4500 psig *

Airbus A380 The Airbus A380 is a large wide-body airliner that was developed and produced by Airbus. It is the world's largest passenger airliner and only full-length double-deck jet airliner. Airbus studies started in 1988, and the project was annou ...

hydraulic system: 5000 psig *

Land Rover Land Rover is a British brand of predominantly four-wheel drive, off-road capable vehicles, owned by multinational car manufacturer Jaguar Land Rover (JLR), since 2008 a subsidiary of India's Tata Motors. JLR currently builds Land Rove ...

Td5 diesel engine fuel injection pressure: 22,500 psi * Ultimate strength of

ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...

A36 steel: 58,000 psi * Water jet cutter: 40,000–100,000 psig

Conversions

The conversions to and from SI are computed from exact definitions but result in a repeating decimal.

P_\text = P_\text \times \frac

P_\text = P_\text \times \frac

As the pascal is a very small unit relative to industrial pressures, the kilopascal is commonly used. 1000 kPa ≈ 145 lbf/in². Approximate conversions (rounded to some arbitrary number of digits, except when denoted by "≡") are shown in the following table.

References

External links

Pressure measurement primer

{{United States Customary Units Units of pressure Customary units of measurement in the United States Imperial units

Multiples

Magnitude

Conversions

See also

References

External links