Units of pressure and their equivalents
| Unit | Symbol | In pascals |
|---|---|---|
| Bar | bar | 100,000 Pa |
| Pound per square inch | psi | 6,895 Pa |
| Pascal | Pa | 1 Pabase unit |
| Kilopascal | kPa | 1,000 Pa |
| Hectopascal | hPa | 100 Pa |
| Standard atmosphere | atm | 101,325 Pa |
| Millimetre of mercury | mmHg | 133.322 Pa |
| Inch of mercury | inHg | 3,386 Pa |
Frequently asked questions
How many psi is 2.3 bar of tyre pressure?
33.4 psi. One bar is 14.5038 psi, and most English-language handbooks round to 33 or 34. The ratio is easy to hold: multiply by 14.5 to go from bar to psi, divide by 14.5 to come back. A pressure of 2.0 bar is 29 psi, 2.5 bar is 36.3 psi, and 3.0 bar is 43.5 psi.
Gauge pressure or absolute pressure?
A tyre gauge measures the difference from the surrounding air, not the real pressure of the gas. A tyre reading 2.3 bar therefore holds air at 3.3 bar absolute, since the atmosphere adds about one. Data sheets sometimes write barg for gauge and bara for absolute, and English tables psig and psia. This converter does not move the origin of the scale: it converts a value, gauge or absolute according to what you type.
Why does weather use hectopascals?
Because the hectopascal is exactly one millibar, the unit used before it. Moving to the SI changed only the name: 1013 hPa today, 1013 mbar yesterday, the same figure on the same chart. Standard atmospheric pressure is 101,325 pascals, that is 1013.25 hPa or 1.01325 bar, and a low below 980 hPa announces a storm.
Should tyres be inflated cold or hot?
Cold, and that is how the manufacturer's figures are given. Driving warms the trapped air, whose pressure rises by about 0.1 bar every ten degrees: a tyre at 2.3 bar on a twenty degree morning reads close to 2.5 after an hour on the motorway. That is not a leak sealing itself, it is Gay-Lussac's law, and bleeding a hot tyre back to the handbook figure leaves it under-inflated once cool.
Why is vacuum measured in absolute millibars?
Because a vacuum is counted from nothing, never from the surrounding air. A garage gauge cannot read below minus one bar anyway, since absolute pressure never turns negative. Vacuum scales therefore start at 1013 millibars, atmospheric pressure: a rotary vane pump reaches one millibar, a turbomolecular pump goes down to 0.0000001 millibar, and freeze-drying works around 0.5 millibar.
Ten tonnes per square metre, and nobody feels them
The pascal is so small that a sheet of paper lying flat exerts barely more than one: eighty grams per square metre weigh 0.78 newton, hence 0.78 pascal. Air, meanwhile, presses on every square metre with 101,325 pascals, the equivalent of ten tonnes. Nothing collapses because the pressure acts in every direction at once and the inside of a body pushes back as hard as the outside. Remove it and it shows itself: a suction cup holds by nothing else.
The millimetre of mercury comes from an experiment by Torricelli in 1643, who upturned a tube full of mercury into a basin and watched the column settle near 760 millimetres. With water, thirteen and a half times less dense, he would have needed a ten metre tube and a balcony. The unit has almost vanished, except in medicine: blood pressure is still quoted in millimetres of mercury the world over, the last stronghold of a four hundred year old scale.