Pressure Converter

Enter a pressure in pascals, kilopascals, megapascals, bar, psi, atmospheres or millimetres of mercury and every other unit follows. Values go through pascals in both directions, and negative numbers are accepted for vacuum and gauge readings.

The distinction this table cannot make for you is absolute versus gauge, and it accounts for most of the errors people make with pressure. A tyre inflated to 32 psi is 32 psi above the air around it — its absolute pressure is about 46.7 psi, because one atmosphere is 14.696 psi and the gauge simply does not count it. An empty tyre reads zero and is very much not at zero pressure. Weather and physics work in absolute; tyres, boilers, dive gear and most industrial instruments read gauge, and the unit symbols rarely say which, though psia and psig exist for exactly this reason. Converting between units never changes which of the two you have.

Two of the units on this list are almost but not quite the same thing. A standard atmosphere is defined as exactly 101,325 pascals, while a bar is exactly 100,000 — so a bar is about 1.3 percent short of an atmosphere, which is enough to matter in a calculation and not enough to notice on a gauge. Weather reports sidestep the question by using hectopascals, which are numerically identical to millibars, so 1013 hPa and 1013 mbar are the same reading written two ways.

Millimetres of mercury carry the same near-miss. The Torr was defined as one 760th of a standard atmosphere, and the millimetre of mercury as the pressure of exactly that much mercury under standard gravity; the two definitions land about one part in seven million apart. For blood pressure, vacuum work and altimetry the difference is invisible, and the names are used interchangeably. It is only in metrology that anyone needs to care which one is written down.