The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in2),[1] is a unit of measurement of pressure or of stress based on avoirdupois units and used primarily in the United States.

It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch.

In SI units, 1 psi is approximately 6,895 pascals.

The pound 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 (101 kilopascals), this will be added to any pressure reading made in air at sea level.

The converse is pound per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure.

[2][3] When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit is pound per square inch differential (psid).

The megapound per square inch (Mpsi) is another multiple equal to a million psi.

It is used in mechanics for the elastic modulus of materials, especially for metals.

The conversions to and from SI are computed from exact definitions but result in a repeating decimal.

As the pascal is a very small unit relative to industrial pressures, the kilopascal is commonly used.

Approximate conversions (rounded to some arbitrary number of digits, except when denoted by "≡") are shown in the following table.