Pressure Converter

Gauge pressure versus absolute pressure

A tyre gauge reading 32 psi means 32 psi above the surrounding air. In absolute terms the tyre contains about 46.7 psi, because atmospheric pressure at sea level is roughly 14.7 psi. Converters work on the numbers you give them, so if you are mixing a gauge reading with an absolute figure — common in diving, refrigeration and vacuum work — you have to add or subtract one atmosphere yourself. Units written “psig” are gauge; “psia” are absolute.

Why weather uses hectopascals and millibars interchangeably

One hectopascal is exactly one millibar. Meteorology switched from millibars to hectopascals when SI units were standardised, and since the numbers are identical nobody had to relearn what 1013 means. American aviation and broadcast weather kept inches of mercury, where standard pressure is 29.92 inHg.

Mercury columns, and why they are still around

A millimetre of mercury is the pressure exerted by a 1 mm column of mercury at 0 °C under standard gravity — 133.322 Pa. It survives in medicine because blood-pressure measurement was built around mercury manometers, and the numbers 120/80 are so deeply embedded that no country has managed to move on. One torr is defined as exactly 1/760 of an atmosphere, which makes it almost but not exactly a millimetre of mercury.

Tyres: the one place a small error is expensive

European cars often specify 2.2 bar, which is 31.9 psi — not 2.2 psi, and not 22 psi. Under-inflating by 20% measurably increases fuel consumption and tyre wear, and badly under-inflated tyres run hot. If a placard gives bar and your gauge gives psi, convert before inflating rather than after.

Frequently Asked Questions