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The pound per square inch or, more accurately, pound-force per square inch 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. In SI units, 1 psi is approximately equal to 6895 Pa.
Pounds per square inch absolute 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 , 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 will have a pressure of 79.7 psia. When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential.