What Does psi and ksi Stand For? (kilo)Pounds per Square Inch, a Unit of Pressure

Yes, I know. Nonmetric unit bad! And nonmetric pressure units? Even worse. 

The abbreviation psi stands for pound-force per square inch. A related unit is ksi, which stands for kilopound-force per square inch.

Most people leave out the “force” part of pound-force, and just say psi is “pounds per square inch.” On earth, the difference is semantics; the pound-force is the force exerted by gravity on 1 pound of mass.

For my non-American audience, a pound is a bit less than half a kilogram, and an inch is about 2.5 centimeters. Converting to pascals (Pa), the SI unit of pressure,

1 psi = 6894.76 Pa

And 1 ksi, which is a thousand psi, is equal to 6894.76 kPa.

The atmospheric pressure is about 15 psi.

1 psi =6894.76 Pa0.068 atm
1 ksi =6,894,760 Pa68 atm

This article was a definition of psi and ksi. If you are interested in all the different, strange units that are used to measure pressure, I wrote an entire article explaining what each unit is, and how to convert between them. Or, if there is a unit you are especially interested in, you can check out one of the other unit definitions I’ve written.

Other Pressure Measurements:
mbar or bar

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