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Pounds per Square Inch (psi)

Also called: psi, lbf/in², pound-force per square inch

Definition: Psi is the US customary unit of pressure: one pound-force acting on one square inch, equal to 6,894.757 pascals or 6.894757 kilopascals.

Updated November 3, 2026 · How we check facts

Psi measures pressure in tire gauges, air compressors and water systems. One psi is one pound-force spread over one square inch. NIST gives 1 psi as 6,894.757 pascals. Many gauges also show bar or kPa.

Formula

kPa = psi × 6.894757
psi = kPa ÷ 6.894757
bar = psi ÷ 14.50377

Example

A tire at 35 psi is 35 × 6.894757 = 241.3 kPa, or 2.413 bar. A 2.5 bar tire is 2.5 × 14.50377 = 36.26 psi. See psi to bar, bar to psi and psi to kPa.

Common mistake

Gauge readings count pressure above the surrounding air; absolute pressure adds the atmosphere. Tire and tool ratings are gauge values.

Related terms

BarThe bar is a metric unit of pressure defined as exactly 100,000 pascals (100 kPa), a little less than standard atmospheric pressure; one bar is 14.50377 psi.Inch (in)The inch is a unit of length equal to exactly 2.54 centimeters (0.0254 m); 12 inches make a foot and 36 make a yard; a meter is 39.37 inches.Acre (ac)The acre is a US customary unit of area equal to exactly 43,560 square feet, 4,840 square yards or 160 square rods; NIST gives it as 4,046.8564224 square meters.Centimeter (cm)The centimeter is a metric unit of length equal to one hundredth of a meter (0.01 m); exactly 2.54 centimeters make one inch.Cubic Foot (ft³)The cubic foot is a volume unit equal to a cube 1 foot on each side: 1,728 cubic inches, 0.02831685 cubic meters, about 28.317 liters or 7.48 US gallons.Cubic Meter (m³)The cubic meter is the SI unit of volume: a cube 1 meter on each side, equal to 1,000 liters, 1,000,000 cubic centimeters, 35.315 cubic feet or 1.308 cubic yards.Cubic Yard (yd³)The cubic yard is a volume unit equal to a cube 1 yard on each side: 27 cubic feet, 46,656 cubic inches, 0.7645549 cubic meters or about 764.55 liters.Fluid Ounce (fl oz)A fluid ounce is a unit of volume; one US fluid ounce is 29.57353 milliliters, and 8 of them make a US cup. It is not the same as the ounce used to weigh things.

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.8 — Factors for Units Listed Alphabetically