Converting tire pressure units
Tire pressure has a big effect on how a vehicle handles, whether it is a BMX bike on dirt jumps or a truck loaded for a trip. Too little pressure makes tires squirm, wear on the edges and waste energy. Too much reduces grip and makes the ride harsh. The trouble is that gauges, pumps and manuals don't all use the same unit, so this converter lets you type a value in whichever unit you have and read it in all the others.
The units
- PSI (pounds per square inch) is standard in the United States and on most bike pumps.
- bar is common in Europe and on many dual-scale gauges. 1 bar is 100 kPa, just under normal atmospheric pressure.
- kPa (kilopascals) is the SI unit, used on door placards in Canada, Australia and much of Asia.
- kgf/cm² (kilogram-force per square centimetre) still appears in Japan and on older gauges. It is almost the same as bar.
- atm (standard atmospheres) is rarely printed on tires but is handy for comparing against air pressure at sea level.
Conversion factors
| 1 unit of | PSI | bar | kPa |
|---|---|---|---|
| PSI | 1 | 0.06895 | 6.895 |
| bar | 14.504 | 1 | 100 |
| kPa | 0.14504 | 0.01 | 1 |
| kgf/cm² | 14.223 | 0.98067 | 98.067 |
| atm | 14.696 | 1.01325 | 101.325 |
A quick mental shortcut: divide PSI by 14.5 to get bar, or multiply bar by 100 to get kPa. So 33 psi is about 2.3 bar, or 228 kPa.
Gauge pressure, not absolute pressure
Tire gauges read pressure above the surrounding air, called gauge pressure. A flat tire reads zero even though the air inside is still at about 1 atm. The numbers here are straight unit conversions, so 2 bar on a gauge is 29 psi on a gauge. You don't need to add or subtract atmospheric pressure.
Getting an accurate reading
- Check car tires cold, before driving or after the car has been parked for a few hours. Warm tires read higher.
- Use the pressure on the door-jamb placard, not the maximum printed on the sidewall.
- Bike pressures depend on tire width, rider weight and terrain. Wider tires run lower pressures, and tubeless setups can usually go lower still.
- Cheap gauges can be a few PSI out. Using the same gauge every time matters more than the brand.
