# Pounds per square inch

A pressure gauge reading in psi (red scale) and kPa (black scale)

The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in2;[1] abbreviations: psi, lbf/sq in, lbf/sq in[citation needed]) 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:

1 psi = ${\displaystyle {\frac {1{\text{ lbf}}}{(1{\text{ in}})^{2}}}}$${\displaystyle {\frac {4.4482{\text{ N}}}{(0.0254{\text{ m}})^{2}}}}$ ≈ 6894.757 N/m²

Therefore, one pound per square inch is approximately 6894.757 Pa.

Now converting the psi to standard atmospheres:

1 atm = ${\displaystyle 101325{\text{ Pa}}}$ = ${\displaystyle {\frac {101325}{6894.757293168}}}$ ≈ 14.70 lbf/in2

Therefore, 1 atmosphere is approximately 14.7 pounds per square inch.

Pounds per square inch absolute (psia) is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi, this will be added to any pressure reading made in air at sea level. The converse is pounds per square inch gauge or pounds per square inch gage (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psi above local atmospheric pressure (say, 14.7 psia locally), will have a pressure of 65 + 14.7 = 79.7 psia or 65 psig.[2][3] 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 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.

The megapound per square inch (Mpsi) is another multiple equal to a million psi. It is used in mechanics for the elastic modulus of the materials, especially for metals.[5]

The conversion in SI Units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.

## Conversions

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 1.450377×10−4
1 bar 105 ≡ 100 kPa

≡ 106 dyn/cm2

1.0197 0.98692 750.06 14.50377
1 at 9.80665×104 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334
1 atm 1.01325×105 1.01325 1.0332 1 760 14.69595
1 Torr 133.3224 1.333224×10−3 1.359551×10−3 ≡ 1/760 ≈ 1.315789×10−3 ≡ 1 Torr

≈ 1 mmHg

1.933678×10−2
1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF /in2