Pressure unit conversion
Car tire labels often show psi and bar side by side — here is how the common pressure units relate.
Unit converter
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| From | To | Multiply by |
|---|---|---|
| 1 bar | kPa | 100 |
| 1 bar | psi | 14.5038 |
| 1 psi | kPa | 6.89476 |
| 1 atm | kPa | 101.325 |
| 1 atm | bar | 1.01325 |
| 1 atm | psi | 14.6959 |
| 1 mmHg (torr) | Pa | 133.322 |
| 1 MPa | bar | 10 |
| Typical car tire | ≈2.2–2.5 bar | ≈32–36 psi |
Notes
- Weather reports may use hPa (1 hPa = 1 mbar).
- Blood pressure mmHg is the same millimeter-of-mercury unit.
How the units relate
The sheet's core conversion is 1 bar = 100 kPa = 14.5 psi ≈ 0.987 atm. The pascal (Pa) is the SI unit: 1 Pa = 1 N/m², and engineers usually work in kilopascals because the pascal itself is tiny. The bar was designed to be close to atmospheric pressure (1 atm = 1.01325 bar = 101.325 kPa), which is why meteorology and European tire gauges favor it. psi — pounds per square inch — is the imperial counterpart: 1 psi = 6.89476 kPa. One megapascal equals 10 bar, so MPa values (boilers, hydraulics) convert by shifting a decimal. Millimeters of mercury (mmHg, or torr) belong to vacuum and blood-pressure work: 1 mmHg = 133.322 Pa, a legacy of the mercury barometer.
Tire-pressure conventions around the world
Tire pressure is where unit confusion bites. Continental Europe and Japan label door-pillar stickers and pump displays in bar (or kPa), while the United States uses psi. A typical car tire sits at 2.2–2.5 bar, which the sheet shows is about 32–36 psi — the same pressure, two numbers. Motorcycles and bicycles run higher (a road bike can exceed 6 bar, over 87 psi), and trucks quote kPa. If a European manual lists 2.3 bar and your US gauge reads psi, multiply by 14.5; going the other way, divide by 14.5 or multiply by roughly 0.069. Never guess: under-inflation heats the tire, over-inflation grips less.
Absolute vs gauge pressure
A gauge reading of 0 does not mean there is no pressure — it means the pressure equals ambient air. Gauges measure pressure relative to the atmosphere (barg, 'bar gauge'), while absolute pressure (bara, or atm) is measured from a perfect vacuum: absolute = gauge + atmospheric (≈1.01325 bar at sea level). This matters everywhere the atmosphere is part of the physics. A tire's 2.3 barg is about 3.3 bara. Boilers, compressors and vacuum systems are quoted in gauge units; gas laws, altitude calculations and pressure ratios need absolute. Mixing them up in a formula can produce errors of one atmosphere — about 14.7 psi — which is fatal in vacuum work and significant elsewhere.
Reading weather maps
Meteorology is the other big everyday use of pressure units. Weather charts report surface pressure in millibars (mb) or hectopascals (hPa) — conveniently, 1 mb = 1 hPa = 100 Pa. Standard sea-level pressure is 1013.25 hPa, which is why '1013' is the reference on every chart: lows below ~1000 hPa bring wind and rain, highs above ~1020 hPa bring settled weather. The same quantity appears in other units on the sheet: 1013.25 hPa = 1013.25 mb = 1.01325 bar = 1 atm = 14.696 psi ≈ 760 mmHg. Altimeters, barometers and station reports all use this one number in different clothes.
Frequently asked questions
- Is bar the same as PSI?
- No. 1 bar equals 14.5038 psi, so multiply bar by about 14.5 to get psi. Both are used for tire pressures.
- How many psi is 2.5 bar?
- 2.5 bar is about 36.3 psi (multiply by 14.5038). This is a typical maximum car tire pressure.
- What is the difference between atm and bar?
- 1 atm (standard atmosphere) is 101.325 kPa, slightly more than 1 bar (100 kPa). They differ by about 1.3%.