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# Pounds per square inch

The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in² or lbf/in²) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch: : pascal (Pa) is the SI unit of pressure.

## Relation to other measures

Other abbreviations are used that append a modifier to "psi". However, the US National Institute of Standards and Technology recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure For example, "Pg = 100 psi" rather than "P = 100 psig".

• psia (pounds-force per square inch absolute) — gauge pressure plus local atmospheric pressure. Replace "x psia" with "Pa = x psi".
• psid (psi difference) — difference between two pressures. Replace "x psid" with "ΔP = x psi".
• psig (pounds-force per square inch gauge). Replace "x psig" with "Pg = x psi".
• psivg (psi vented gauge) — difference between the measuring point and the local pressure. Replace "x psivg" with "Pvg = x psi".
• psisg (psi sealed gauge) — difference between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point. Replace "x psisg" with "Psg = x psi".

### psig

Psig (pound-force per square inch gauge) is a unit of pressure relative to the surrounding atmosphere. By contrast, psia measures pressure relative to a vacuum (such as that in space).

At sea level, Earth's atmosphere actually exerts a pressure of 14.7 psi (see below). Humans do not feel this pressure because internal pressure of liquid in their bodies matches the external pressure. If a pressure gauge is calibrated to read zero in space, then at sea level on Earth it would read 14.7 psi. Thus a reading of 30 psig on a tire gauge represents an absolute pressure of 44.7 psi.

Psi is often used incorrectly instead of psig.

### ksi and kip

The ksi (kip, "kilo-pound[-force] per square inch") is 1000 psi, combining the prefix kilo with the psi abbreviation. It is occasionally used in materials science and mechanical engineering to specify stress and Young's modulus.

## Magnitude

• Atmospheric pressure at sea level (standard): Pa = 14.7 psi
• Automobile tire overpressure (common): Pg = 32 psi
• Bike tire overpressure (common): Pg = 65 psi
• Air brake (rail) or Air brake (road vehicle) reservoir overpressure (common): 90 psi ≤ Pg ≤ 120 psi
• Road racing bicycle tire overpressure: Pg = 120 psi
• Full SCBA Self Contained Breathing Apparatus for toxic atmospheres: 2216 psi
• Ivan Drago's punch: 1850 to 2150 psi
• Full scuba tank overpressure (common): Pg = 3,000 psi
• Commercial jet airliner hydraulic pressure: 3000 psi
• Airbus A380 hydraulic system: 5000 psi