Convert psi to bar

One psi equals 0.0689476 bar. The reverse is the number worth memorizing: one bar is about 14.5 psi. Both resolve to the pascal, the SI unit of pressure — one bar is exactly 100,000 pascals and one psi is 6,894.757 pascals.

2.2 bar is 31.9 psi. European cars print the recommended pressure in bar on the door jamb while most pumps read in psi, which is why this is the conversion drivers actually need:

Tyre pressure is usually checked on the same trip as fuel — gallons to liters covers the other number on the forecourt.

The Norwegian meteorologist Vilhelm Bjerknes introduced it in 1911, from the Greek baros, weight. He chose it to sit near average atmospheric pressure at sea level — 1.01325 bar — so weather maps could use round numbers. The bar is not an SI unit, though the International Bureau of Weights and Measures lists it among units accepted for use alongside the SI. psi is plainer: pounds of force per square inch.

This is where the conversion goes wrong even when the arithmetic is right. A tire gauge reads gauge pressure — how much the inside exceeds the surrounding air. Absolute pressure adds the atmosphere back, about 1 bar or 14.7 psi at sea level, so a tire reading 2.2 bar on the gauge is at 3.2 bar absolute.

Converting between psi and bar does not change which of the two you have. Before comparing a figure from a manual against one from a gauge, check that both are gauge pressure. The notations psig and psia exist for exactly this reason.

NIST Special Publication 811, the US guide to the SI gives one pound-force per square inch as 6,894.757 Pa. The bar is exactly 100,000 Pa; it is not an SI unit, while the pascal — one newton per square metre — is the SI unit of pressure defined in the SI Brochure published by the International Bureau of Weights and Measures (BIPM).