Pressure units — psi, bar, atm, pascal, torr
Why every industry has its own pressure unit and how to translate between them.
Pressure has the largest variety of common units of any physical quantity on this site. That's because pressure matters in many industries, and each picked its favourite.
The SI base
The SI unit is the pascal (Pa), which is 1 newton per square metre. Atmospheric pressure is 101,325 Pa.
Practical units
- bar: 100,000 Pa. Diving, hydraulics, brewing. - psi (pounds per square inch): 6,894.76 Pa. US tire pressure, plumbing. - atm (atmosphere): 101,325 Pa. Old physics default, still used in chemistry ("Standard Temperature and Pressure"). - mmHg (millimetres of mercury): 133.322 Pa. Medical blood pressure. - torr: 1 mmHg exactly. Vacuum science; named after Torricelli. - mbar (millibar): 100 Pa. Meteorology and altimeters.
A useful comparison
| Unit | Pa | A car tire? | |---|---|---| | pascal (Pa) | 1 | no — too small | | kilopascal (kPa) | 1,000 | 220 kPa = 32 psi (typical car) | | bar | 100,000 | 2.2 bar = 32 psi | | psi | 6,894.76 | 32 psi = 220 kPa | | atm | 101,325 | 2.17 atm = 32 psi |
Why each unit exists
- bar was invented by Norwegian meteorologist Vilhelm Bjerknes in 1909 because the millibar was a clean unit for atmospheric data. - atm is the natural unit if you think in terms of "what fraction of an atmosphere". - psi sticks around because US engineering was built on inch-pound-second units (before SI was complete). - mmHg is still in medical practice because the sphygmomanometer literally uses a column of mercury.
Which do you need?
If you're in the US measuring a tire, psi. In Europe, bar. In aviation, kPa or inHg. For scientific work, Pa. For blood pressure, mmHg. The site lets you convert any of these to any other.