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The pound-force per square inch (symbol: lbf/in²) is a non-SI unit of pressure based on avoirdupois units. In casual English language use it is rendered as "pounds per square inch", abbreviated to psi with little distinction between "mass" and "force".

In Iran and some other countries, technicians refer to it as "Pound" which is incorrect, but common. (e.g. the pressure of gas line is about 0.25 pound which means the line has the pressure of 0.25 psi.)

Contents 1 Definition 1.1 KSI and Kip 2 Context 3 See also 4 External links

At 1 lbf/in², a force of one pound-force is applied to an area of one square inch.

1 lbf/in² = 6,894.75729 pascals (Pa)

Other abbreviations are used that append a modifier to "psi". However, the 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[1] For example, "P_g = 100 psi" rather than "P = 100 psig".

• psia (pounds-force per square inch absolute) - gauge pressure plus local atmospheric pressure. Replace "x psia" with "P_a = 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 "P_g = x psi".
• psivg (psi vented gauge) - difference between the measuring point and the local pressure. Replace "x psivg" with "P_vg = 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 "P_sg = x psi".

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

• Atmospheric pressure at sea level: P_a = 14.7 psi
• Automobile tire: P_g = 32 psi
• Bike tire: P_g = 65 psi
• Air brake reservoir: 90 psi ≤ P_g ≤ 120 psi
• Full scuba tank: P_g = 3,000 psi

Pressure Units

	Pascal (Pa)	Bar (bar)	Technical atmosphere (at)	Atmosphere (atm)	Torr (mmHg)	Pound-force per square inch (psi)
1 Pa	≡ 1 N/m²	10−5	10.197×10−6	9.8692×10−6	7.5006×10−3	145.04×10−6
1 bar	100 000	≡ 106 dyn/cm²	1.0197	0.98692	750.06	14.504
1 at	98 066.5	0.980665	≡ 1 kgf/cm²	0.96784	735.56	14.223
1 atm	101 325	1.01325	1.0332	≡ 1 atm	760	14.696
1 torr	133.322	1.3332×10−3	1.3595×10−3	1.3158×10−3	≡ 1 mmHg	19.337×10−3
1 psi	6 894.76	68.948×10−3	70.307×10−3	68.046×10−3	51.715	≡ 1 lbf/in²

Example reading: 1 Pa = 1 N/m²  = 10⁻⁵ bar  = 10.197×10⁻⁶ at  = 9.8692×10⁻⁶ atm ....etc.
Note: mmHg is an abbreviation for millimetres of mercury.

• Conversion of units
• Pressure

• Pressure measurement primer