Tire Pressure Explained: PSI, Bar and kPa
Tire pressure is one of the few measurements most drivers meet every month, and it is also a case where units change from country to country. American cars and gauges use psi, European cars use bar and many Asian, Canadian and Australian vehicles use kilopascals. Mixing them up can leave a tire dangerously low, so here is a plain guide.
The three units
- psi (pounds per square inch) is the pressure from one pound of force on one square inch. Typical cars run at 30 to 36 psi.
- bar is exactly 100,000 pascals, close to atmospheric pressure. Typical cars run at 2.0 to 2.5 bar.
- kPa (kilopascals) is the SI-based unit. Typical cars run at 200 to 250 kPa.
Quick conversion table
| psi | bar | kPa |
|---|---|---|
| 28 | 1.93 | 193 |
| 30 | 2.07 | 207 |
| 32 | 2.21 | 221 |
| 35 | 2.41 | 241 |
| 36 | 2.48 | 248 |
| 40 | 2.76 | 276 |
| 60 | 4.14 | 414 |
| 100 | 6.89 | 690 |
For exact values, use the bar to psi, psi to bar and kilopascals to psi converters.
Which number do I follow?
Look for the tire placard on the driver’s door jamb, in the fuel flap or in the owner’s manual. It lists the recommended cold pressure for front and rear tires, and sometimes for a heavy load. Do not use the number molded into the tire sidewall: that is the maximum the tire can hold, not the recommended pressure.
Why the right pressure matters
- Safety: an under-inflated tire flexes too much, overheats and can fail at speed. An over-inflated tire has a smaller contact patch and less grip, especially in the wet.
- Fuel economy: low pressure increases rolling resistance, which costs fuel.
- Wear: too little pressure wears the edges of the tread, while too much wears the center.
Check them cold
Pressure rises as tires warm up while driving, typically by 2 to 4 psi. Always check them when the car has been parked for a few hours, and adjust to the placard figure. Check the spare, too.
Gauge pressure vs absolute pressure
Tire gauges show gauge pressure, which is the pressure above the surrounding atmosphere. Absolute pressure adds about 14.7 psi (1.01 bar) on top. All the tire figures above are gauge pressure, so you do not need to add anything.