The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in^{2} or lb_{f}/in^{2} or lbf/sq in or lb_{f}/sq in) 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:

Using unit conversion:

1 psi = ≈ 6894.757293168 N/m²

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

Therefore, 1 atmosphere is approximately 14.696 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 atmospheric pressure, will have a pressure of 65 + 14.7 = 79.7 psia or 65 psig.^{}^{}

The kilopound per square inch is a scaled unit derived from psi, equivalent to a thousand psi.

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.