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 and e ...

or of

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one

square inch A square inch (plural: square inches) is a unit of area, equal to the area of a square with sides of one inch. The following symbols are used to denote square inches: *square in *sq inches, sq inch, sq in *inches/-2, inch/-2, in/-2 *inches^2, inc ...

. 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 di ...

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 standardise ...

. 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 t ...

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 object ...

for the

elastic modulus An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...

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: 0.036 psid * Blood pressure – clinically normal human blood pressure (120/80 mmHg): 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 carbo ...

residential piped in for consumer appliance; 4–6 psig. * Boost pressure provided by an automotive

turbocharger In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

(common): 6–15 psig *

NFL football The National Football League (NFL) is a professional American football league that consists of 32 teams, divided equally between the American Football Conference (AFC) and the National Football Conference (NFC). The NFL is one of the major ...

: 12.5–13.5 psig *

Atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...

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) A railway air brake is a railway brake power braking system with compressed air as the operating medium. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on April 13, 1869. The W ...

air brake (road vehicle) An air brake or, more formally, a compressed-air-brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad or brake shoe needed to stop the vehicle. Air bra ...

reservoir overpressure (common): 90–120 psig * Road racing bicycle tire overpressure: 120 psig * Steam locomotive

fire tube boiler A fire-tube boiler is a type of boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating appl ...

(UK, 20th century): 150–280 psig *

Union Pacific Big Boy The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962. The 25 Big ...

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 A self-contained breathing apparatus (SCBA), sometimes referred to as a compressed air breathing apparatus (CABA) or simply breathing apparatus (BA), is a device worn to provide breathable air in an atmosphere that is immediately dangerous to ...

(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 (self-contained underwater breathing apparatus) tank overpressure (common): 3000 psig * Full

(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 Rovers ...

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

Ultimate 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 t ...

of ASTM

A36 steel A36 steel is a common structural steel alloy utilized in the United States.''Steel Construction Manual'', 8th Edition, second revised edition, American Institute of Steel Construction, 1986, Ch. 1 pp. 1–5. The A36 (UNS K02600) standard was establ ...

: 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