pound per square inch

A unit of pressure, now used mostly only in the United States. The pound in this term is a unit of force, not mass. 1 pound per square inch is about 6894.757 pascals; see the converter. Abbreviation, psi. In practice, it is often necessary to say whether a measurement of pressure does or does not include atmospheric pressure. To do this two more clearly defined units are available:

pressure gauge

©iStockphoto.com/Yegor Korzh

Pounds per square inch gauge (abbreviation, psig) are the units you are measuring in when you measure the pressure in an automobile tire. You are actually measuring the difference between the pressure inside the tire and the pressure outside (atmospheric pressure). Most items marked in psi, such as tires, are really measured in psig. When people speak of psi, they usually mean psig.

A measurement in pounds per square inch absolute (abbreviation, psia) is about 14.7 more than a measurement in psig depending on what the atmospheric pressure was when the measurement in psig was made.

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