What the pressure units measure
Pressure is force spread over an area. The SI unit is the pascal, one newton on one square meter, which is tiny, so you usually see kilopascals, megapascals or bar. The others are older or tied to a trade.
- 1 bar = 100,000 Pa = 100 kPa, exactly. A millibar is a hundredth of that, and equals a hectopascal.
- 1 standard atmosphere (atm) = 101,325 Pa = 1.01325 bar = 760 torr. It is a defined average of sea level air pressure, not today's reading.
- 1 psi = 6.894757 kPa = 0.0689476 bar, so 1 bar is 14.5038 psi and 1 atm is 14.6959 psi.
- 1 technical atmosphere is one kilogram-force per square centimeter, 98,066.5 Pa or 0.980665 bar.
- 1 mmHg is the pressure of a millimeter column of mercury, 133.322387 Pa, and 1 inHg is 25.4 mmHg.
The psi to bar page goes into the most requested pair, with a table of common values.
Gauge pressure and absolute pressure
A tyre gauge, a boiler gauge and most pressure washers show gauge pressure: the amount above the air around them. A flat tyre reads 0 on the gauge although there is air at about 101 kPa inside it. Absolute pressure counts from a perfect vacuum, so it is the gauge reading plus the pressure of the air. At sea level that is about 14.7 psi, so 30 psi gauge is about 44.7 psi absolute.
Engineers write psig for gauge and psia for absolute. A unit converter changes the unit, not the kind of reading, so convert 30 psi gauge to 2.07 bar gauge, not to 2.07 bar absolute. Weather pressure and barometers are absolute, and so are most pressure values in physics and chemistry.
Where each unit turns up
- psi on US tyres, compressors, pressure washers and scuba tanks.
- bar and kPa on tyres in most other countries, in hydraulics and in diving. Tyre placards often list kPa as well as psi.
- hPa and mbar on weather maps. Standard sea level pressure is 1,013.25 hPa.
- inHg in US weather reports and aircraft altimeter settings, where standard pressure is 29.92 inHg.
- mmHg in medicine. A blood pressure of 120/80 mmHg is about 16.0 and 10.7 kPa.
- Torr in vacuum technology, and atm in chemistry.
mmHg, torr and inHg are close, not identical
A torr was meant to be a millimeter of mercury, but it is now defined as exactly 1/760 of a standard atmosphere, which is 133.322368 Pa. The conventional millimeter of mercury is 133.322387 Pa. They differ by about 0.000014%, which is invisible in practice but real, so the converter keeps them as two units. The inch of mercury here is exactly 25.4 conventional mmHg and is based on mercury at 0 °C. Real mercury columns change with temperature and gravity, so instruments are corrected to these conventions.