Pressure converter

Convert to pascals, then to the target unit. A tire at 32 psi: 32 × 6,894.757 = 220,632 Pa, which is 220.6 kPa or 2.21 bar. The pascal is tiny — roughly the pressure of a sheet of paper resting on a table — so everyday pressures run into thousands or hundreds of thousands, and kilopascals, bars and hectopascals exist to keep the numbers short.

Because one hectopascal equals one millibar, the unit meteorologists used for decades, so the switch to SI did not change a single number on the map. Standard sea-level pressure is 1013.25 hPa. The bar itself, 100,000 Pa, was chosen to sit close to atmospheric pressure; it is not an SI unit, but it survives in tire gauges, diving and industry.

Because the first instruments that measured pressure were columns of mercury. Evangelista Torricelli built the mercury barometer in 1643–44, and the height of the column became the unit. The torr, named after him, is 1/760 of a standard atmosphere and differs from the conventional millimeter of mercury by less than one part in seven million. Medicine still records blood pressure in mmHg, which is why a reading of 120 means 120 millimeters of mercury, about 16 kPa.

Most gauges show how far a pressure is above the surrounding air, not above a vacuum. A tire reading 2.2 bar on the gauge holds about 3.2 bar in absolute terms. Converting units does not change which kind of pressure you have, so check that both numbers are the same kind before comparing them. The conversions people need most here are psi to bar and bar to psi.