psi to bar Converter

The figures

The bar is defined as exactly 100,000 pascals, and the pascal is one newton per square metre. The bar is not strictly an SI unit but it survives because one bar is very close to average atmospheric pressure at sea level, which makes it intuitive: two bar in a tyre is roughly twice atmospheric.

Psi — pounds per square inch — derives from pound-force and the inch, so its pascal equivalent is an awkward number. A serviceable mental rule is that 1 bar is about 14.5 psi, so 2 bar is about 29 psi and 2.5 bar about 36 psi.

Tyre pressures worth recognising

Most passenger cars run between 30 and 35 psi, which is 2.1 to 2.4 bar or 210 to 240 kPa. Vans and loaded vehicles run higher, often 45 to 55 psi. Road bicycle tyres run far higher, commonly 80 to 110 psi, while mountain bike tyres run far lower, sometimes 20 to 30.

The correct figure for a car is on a placard in the driver's door jamb or in the fuel filler flap, and it is not the number moulded into the tyre sidewall — that is the tyre's maximum, not the vehicle's recommendation. Inflating to the sidewall figure is a common mistake that produces a harsh ride and uneven wear.

Gauge pressure versus absolute

A tyre gauge reads zero when connected to open air, even though the air around it is at roughly one bar. What it measures is pressure above atmospheric, called gauge pressure. The true absolute pressure inside a tyre at 32 psi gauge is about 46.7 psi absolute.

For inflating tyres this never matters, because every gauge works the same way. It matters a great deal in engineering, meteorology and diving, where absolute pressure is what governs the physics. Units are sometimes written psig and psia, or barg and bara, to make the distinction explicit — and where a figure carries no suffix in an everyday context, gauge is the safe assumption.

Temperature changes pressure

Tyre pressure rises as the tyre heats up, by roughly 1 psi for every 10°F of temperature increase, which is about 0.07 bar per 5°C. Driving warms tyres considerably, so a reading taken after a journey will be higher than the same tyre measured cold.

Recommended pressures are always cold pressures, meaning the vehicle has been stationary for a few hours or driven less than a mile or two. Bleeding air out of a hot tyre to reach the placard figure leaves it underinflated once it cools, which is worse than the mild overinflation it was meant to correct. Seasonal change matters for the same reason: a tyre set correctly in summer will read low in winter without losing any air at all.

Frequently asked questions