What it does
This converts tire pressure between psi, bar, kPa and kgf/cm². It helps when your car's door placard, your gauge, a pump at a gas station or a European or Asian vehicle manual uses a different unit than you're used to.
Inflate to the pressure on your vehicle's placard, not the number on the tire. NHTSA says the correct pressure is the one the vehicle manufacturer lists on the Tire and Loading Information label, usually on the driver's door edge, or in the owner's manual. The number molded on the tire sidewall is the tire's maximum, not the pressure to use. Check pressure "cold", when the car hasn't been driven for at least three hours, and at least once a month, including the spare.
How it's calculated
Every unit is converted through the pascal (Pa), using NIST's conversion factors:
1 psi = 0.45359237 kg × 9.80665 m/s² ÷ (0.0254 m)² = 6,894.757293… Pa
The psi factor is built from exact definitions: the pound is exactly 0.45359237 kg, standard gravity is exactly 9.80665 m/s², and the inch is exactly 2.54 cm. NIST rounds it to 6.894 757 kPa; this calculator uses the full value.
Worked example
Your placard says 2.4 bar. 2.4 × 100,000 = 240,000 Pa, and 240,000 ÷ 6,894.757 = 34.81 psi, so set your gauge to about 35 psi. In the other direction, 35 psi × 6.894757 = 241.3 kPa.
Common tire pressures
| psi | bar | kPa | kgf/cm² |
|---|---|---|---|
| 26 | 1.79 | 179 | 1.83 |
| 28 | 1.93 | 193 | 1.97 |
| 30 | 2.07 | 207 | 2.11 |
| 32 | 2.21 | 221 | 2.25 |
| 34 | 2.34 | 234 | 2.39 |
| 36 | 2.48 | 248 | 2.53 |
| 38 | 2.62 | 262 | 2.67 |
| 40 | 2.76 | 276 | 2.81 |
| 42 | 2.90 | 290 | 2.95 |
| 44 | 3.03 | 303 | 3.09 |
Limitations
- These are gauge pressures, the reading above the surrounding air pressure, which is what tire gauges show.
- Gauges differ. A cheap gauge can read a few psi off, so compare yours with a good one now and then.
- Pressure rises as tires warm up. A reading taken after driving will be higher than the cold pressure the placard means, so don't let air out of warm tires to match it.