Convert bars to psi

The formula

psi = bars × 14.503774

bars to psi — quick reference

A European placard read against an American forecourt gauge

The sticker inside the driver’s door of a European car quotes bar: around 2.2 front and 2.4 rear on a typical saloon, higher with a loaded boot or a full car. The air line at an American forecourt reads psi, as does most inexpensive pump equipment sold worldwide. The same tyres want about 32 psi and 35 psi, and neither device translates.

The error people make is not a small one. Guessing at a factor of ten turns 2.2 bar into 22 psi, which is dangerously soft — a tyre at that pressure runs hot, wears its shoulders, costs measurable fuel and risks a blowout on a long motorway run. Under-inflation is the more common failure because tyres lose pressure slowly and nothing announces it.

Gauges read pressure above atmospheric, not absolute

A tyre gauge measures the difference between the air inside the tyre and the air around it, which is why a completely flat tyre reads zero rather than the roughly 1 bar of atmosphere pressing on it from outside. The conversion holds in either convention, but the reference point has to match on both sides of any comparison.

Two practical consequences follow. Pressures must be checked cold: a tyre driven for twenty minutes reads perhaps 0.3 bar high, and bleeding that off leaves it under-inflated once it cools. Altitude matters slightly too, since atmospheric pressure falls as you climb, so a gauge reads a little higher on a mountain pass than at sea level with identical air inside.

Questions