Pressure is force spread over area, and almost every field that uses it picked a different unit. Weather uses hectopascals or inches of mercury, tyres use psi or bar, medicine uses millimetres of mercury, and engineering uses pascals. They all measure the same thing.
Quick reference
The table follows whichever units you select above.
Anchors worth knowing
1 atm = 101,325 Pa = 1,013.25 hPa = 14.696 psi— sea-level average1 bar = 100,000 Pa— near enough one atmosphere for most purposes1 psi ≈ 6,895 Pa- Car tyres: roughly 32 psi, 2.2 bar, or 220 kPa
- Blood pressure: about 120/80 mmHg
Gauge pressure versus absolute
This is the distinction that causes real errors. A tyre gauge reading 32 psi means 32 psi above the surrounding air. In absolute terms the tyre holds about 46.7 psi, because the atmosphere is already pushing at 14.7 psi.
Most everyday readings — tyres, bike pumps, air compressors — are gauge pressure, sometimes written psig. Scientific and weather figures are absolute. Converting between units does not convert between these two reference points, so check which you have before comparing.
Why mercury? Both mmHg and inHg come from the mercury barometer, where pressure was read as the height of a column the air could support. A millimetre of mercury is now defined exactly as 133.322387415 pascals, so the unit survives its instrument.
Where people go wrong
Confusing bar and psi on a pump
They differ by a factor of about 14.5. A road bike tyre wanting 8 bar needs 116 psi; setting 8 psi instead leaves it essentially flat, and aiming for 116 bar is not physically available from a hand pump.
Millibars and hectopascals
These are the same size — 1 mbar = 1 hPa exactly. Weather services switched labels decades ago without changing the numbers, so an old chart in millibars and a new one in hectopascals are directly comparable.
Common questions
How many psi in a bar?
About 14.504. So 2 bar is roughly 29 psi, a typical car tyre.
What is normal atmospheric pressure?
1,013.25 hPa at sea level by definition, which is 29.92 inHg or 14.696 psi. Actual readings vary by a few per cent with weather and altitude.
Is 1 bar the same as 1 atmosphere?
Almost. A bar is exactly 100,000 Pa and an atmosphere is 101,325 Pa, so a bar is about 1.3 per cent smaller. Close enough for tyres, not for calibration.