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Pressure Converter
Convert psi, bar, kilopascals, atmospheres and mmHg for tyres, gauges and weather.
Your car’s handbook says 2.2 bar. The gauge on the garage forecourt reads in psi. Neither figure is wrong, they are just two different scales for measuring the same squeeze of air, and mixing them up on a tyre is an easy way to over- or under-inflate it.
One atmosphere, five ways of writing it
Standard atmospheric pressure at sea level is the anchor point for the whole system: 101.325 kPa, 1.01325 bar, 14.696 psi, 760 mmHg, and 760 torr are all the exact same pressure, just measured with different instruments and traditions. Weather forecasters use hectopascals or millibars, divers and engineers use bar or atm, American car tyres use psi, and older medical and scientific equipment still reads in mmHg.
The mercury-based units trace back to Evangelista Torricelli, who built the first barometer in 1643 by inverting a mercury-filled tube in a dish. The 760 mm the mercury column stood at under normal sea-level pressure became the definition of one atmosphere long before anyone had settled on a metric system to compare it against.
Pressure units at one atmosphere
| Unit | Value at 1 atm | Common context |
|---|---|---|
| Pascal / kPa | 101,325 Pa (101.325 kPa) | SI unit, engineering and physics |
| Bar | 1.01325 bar | Diving, weather, European tyre gauges |
| psi | 14.696 psi | US and UK tyre pressures, plumbing |
| mmHg / torr | 760 mmHg | Blood pressure, vacuum systems, older barometers |
| Atmosphere (atm) | 1 atm | Chemistry and reference standard |
Converting a tyre pressure correctly
- Check your handbook or door-jamb sticker for the recommended pressure and note its unit.
- Enter that figure into the converter with the matching unit selected.
- Read off the psi value if your pump gauge is calibrated in psi.
- Round to the nearest whole psi — pumps rarely resolve finer than that anyway.
- Re-check cold, since pressure rises as tyres warm up from driving.
Pressure conversion questions
How many psi is 2.2 bar?
2.2 bar converts to about 31.9 psi. This is a common passenger car tyre pressure, and the two figures are often printed side by side on tyre placards precisely because different countries’ pumps use different units.
What is the difference between bar and atmosphere?
They are nearly identical: 1 atm equals 1.01325 bar. For everyday purposes, such as rough tyre or diving-gauge checks, the two can be treated as interchangeable, though precise scientific work keeps them distinct.
Why is blood pressure measured in mmHg?
Because early sphygmomanometers used a literal column of mercury, the same principle as Torricelli’s barometer. The unit stuck even after most devices switched to digital sensors, so a reading like 120/80 still means millimetres of mercury.
What is the difference between mmHg and torr?
For practical purposes, nothing — 1 torr is defined as 1/760 of a standard atmosphere, which makes it numerically almost identical to 1 mmHg. The tiny difference (less than 0.001%) never matters outside of metrology labs.
Why do UK and US tyre gauges show different numbers for the same tyre?
They are reading the same pressure in different units, not different pressures. A UK gauge in bar or kPa and a US gauge in psi will show different numbers for an identical, correctly inflated tyre — convert before comparing, never compare the raw numbers.
