The units of pressure
Pressure is force spread over an area, and the SI unit is the pascal: one newton per square metre. A pascal is tiny, so practical units are all multiples of it. The bar is exactly 100,000 pascals and was chosen to sit near atmospheric pressure; psi comes from pounds per square inch; and millimetres of mercury survive in medicine because the first barometers were literally columns of mercury.
| Unit | Symbol | Equals |
|---|---|---|
| Pascals | Pa | 1 Pa |
| Kilopascals | kPa | 1,000 Pa |
| Bars | bar | 100,000 Pa |
| Pounds per square inch | psi | 6,894.75729317 Pa |
| Atmospheres | atm | 101,325 Pa |
| Millimetres of mercury | mmHg | 133.32238742 Pa |
How to convert by hand
Every pressure conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Convert to pascals and back out. The three numbers worth knowing: a bar is 100,000 Pa, a psi is 6,895 Pa, and standard atmospheric pressure is 101,325 Pa.
The equivalents people look up most
| Conversion | Result |
|---|---|
| Atmospheric pressure | 1.013 bar · 14.7 psi · 760 mmHg |
| A car tyre | 2.2 bar · 31.9 psi |
| Blood pressure, systolic | 120 mmHg · 0.16 bar |
| An espresso machine | 9 bar · 130.5 psi |
| A bicycle tyre | 6 bar · 87 psi |
Common mistakes
- Confusing gauge pressure with absolute pressure. A tyre at 2.2 bar on the gauge is at 3.2 bar absolute, because the gauge reads the difference from the atmosphere.
- Treating bar and atmosphere as identical. They differ by 1.3%, which matters in a laboratory and not on a tyre.
- Measuring tyre pressure hot. Driving raises it by several psi and hides an underinflated tyre.
- Mixing up psi and psig. The trailing g means gauge pressure, and most tyre gauges read psig without saying so.