| Pound per square inch | |
|---|---|
 Apressure gauge reading in psi (red scale) andkPa (black scale) | |
| General information | |
| Unit system | Imperial units,US customary units |
| Unit of | Pressure,stress |
| Symbol | psi, lbf/in2 |
| Conversions | |
| 1 psiin ... | ... is equal to ... |
| SI units | ≈6894.757 Pa |
Thepound per square inch (abbreviation:psi) or, more accurately,pound-force per square inch (symbol:lbf/in2),[1] is aunit of measurement ofpressure or ofstress based onavoirdupois units and used primarily in the United States. It is the pressure resulting from aforce with magnitude of onepound-force applied to an area of onesquare inch. InSI units, 1 psi is approximately 6.895kilopascals.
Thepound per square inch absolute (psia) is used to make it clear that the pressure is relative to avacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi (101kilopascals), this will be added to any pressure reading made in air atsea level. The converse ispound 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).[2][3] When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit ispound per square inch differential (psid).
Thekilopound 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 inmaterials science, where thetensile strength of a material is measured as a large number of psi.[4]
The conversion in SI units is 1 ksi = 6.895 MPa, or 1 MPa = 0.145 ksi.
Themegapound per square inch (Mpsi) is another multiple equal to a million psi. It is used inmechanics for theelastic modulus of materials, especially for metals.[5]
The conversion in SI units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.
The conversions to and from SI are computed from exact definitions but result in a repeating decimal.[6][7]
Approximate conversions (rounded to some arbitrary number of digits, except when denoted by "≡") are shown in the following table.
| Pascal | Bar | Technical atmosphere | Standard atmosphere | Torr | Pound per square inch | |
|---|---|---|---|---|---|---|
| (Pa) | (bar) | (at) | (atm) | (Torr) | (psi) | |
| 1 Pa | — | 10−5 bar | 1.0197×10−5 at | 9.8692×10−6 atm | 7.5006×10−3 Torr | 0.000145037737730 lbf/in2 |
| 1 bar | 105 | — | =1.0197 | =0.98692 | =750.06 | =14.503773773022 |
| 1 at | 98066.5 | 0.980665 | — | 0.9678411053541 | 735.5592401 | 14.2233433071203 |
| 1 atm | ≡101325 | ≡1.01325 | 1.0332 | — | ≡ 760 | 14.6959487755142 |
| 1 Torr | 133.322368421 | 0.001333224 | 0.00135951 | 1/760 ≈0.001315789 | — | 0.019336775 |
| 1 psi | 6894.757293168 | 0.068947573 | 0.070306958 | 0.068045964 | 51.714932572 | — |
A metal's yield strength and ultimate tensile strength values are expressed in tons per square inch, pounds per square inch or thousand pounds (KSI) per square inch. For example, a tensile strength of a steel that can withstand 40,000 pounds of force per square inch may be expressed as 40,000 PSI or 40 KSI (with K being the denominator for thousands of pounds). The tensile strength of steel may also be shown in MPa, or megapascal.