Tire Pressure Units: PSI, Bar, kPa and When to Check
Converting between the units is the easy part. Almost every wrong tire pressure comes from reading the wrong number in the first place, or reading the right number at the wrong time.
Reference only. Follow the pressure your vehicle manufacturer specifies, and if a tire is damaged or losing pressure repeatedly, have it inspected rather than topped up.
Use the placard, not the sidewall
The number moulded into the tire sidewall, usually somewhere between 44 and 51 psi, is the maximum pressure that tire is built to hold. It is a structural limit, not a recommendation, and it is a property of the tire rather than of your car.
The number you want is on the vehicle placard: a sticker inside the driver's door jamb, sometimes in the glovebox or on the fuel filler flap. It is set by the manufacturer for that vehicle's weight and suspension, and it is typically 30 to 35 psi for a passenger car, well below the sidewall figure.
Inflating to the sidewall maximum gives a harsh ride, reduces grip, and wears the center of the tread while the shoulders stay untouched. It is the single most common tire pressure mistake.
Check cold, not hot
Driving heats the tire, and heat raises pressure. A reading taken after a motorway run typically sits 4 to 6 psi above the same tire's cold value.
Placard figures assume cold tires, which means the vehicle has been parked at least three hours, or driven less than about a mile. The trap is arriving at a garage with hot tires, seeing a number above the placard figure, and bleeding air out to match. Once the tires cool, they are several psi underinflated.
If you can only check hot, inflate to the placard figure anyway and accept that it will read slightly high until it cools. Never let air out of a hot tire to reach a cold-tire target.
Which unit is your gauge in
Four units are in circulation, and which one you meet depends on where the gauge and the vehicle came from.
- psi — pounds per square inch. Standard in North America.
- bar — standard across most of Europe. One bar is 100 kPa exactly.
- kPa — kilopascals. Standard in Australia and used in many owner's manuals worldwide.
- kgf/cm² — kilogram-force per square centimeter, found on older Asian gauges. Close enough to bar that the two are used interchangeably in practice; the difference is about 2 percent.
A European placard reading 2.2 and a North American one reading 32 are describing nearly the same pressure. If a gauge shows a number around 2, it is almost certainly bar. Around 30, psi. Around 220, kPa.
Conversion table
| psi | kPa | bar | kgf/cm² |
|---|---|---|---|
| 26 | 179 | 1.79 | 1.83 |
| 28 | 193 | 1.93 | 1.97 |
| 30 | 207 | 2.07 | 2.11 |
| 32 | 221 | 2.21 | 2.25 |
| 33 | 228 | 2.28 | 2.32 |
| 35 | 241 | 2.41 | 2.46 |
| 36 | 248 | 2.48 | 2.53 |
| 38 | 262 | 2.62 | 2.67 |
| 40 | 276 | 2.76 | 2.81 |
| 44 | 303 | 3.03 | 3.09 |
For any other value, use the kPa to psi converter, or the pressure converter if your gauge is in bar or kgf/cm².
Why the warning light comes on in winter
Pressure moves roughly 1 psi for every 10 degrees Fahrenheit of ambient temperature change. A tire set correctly at 32 psi on a warm September afternoon can read 28 or lower on the first hard frost.
Nothing has leaked. The air has contracted. This is why low pressure warnings arrive in clusters on the first cold morning of the season, and why pressures set in summer are worth rechecking when the weather turns.
What TPMS does and doesn't tell you
In the United States, FMVSS 138 requires a tire pressure monitoring system to light the warning within 20 minutes of a tire reaching 25 percent below the placard pressure, or a minimum activation pressure set in the standard, whichever of the two is higher. On a 32 psi car the 25 percent figure is 24 psi, which is already meaningfully underinflated.
A TPMS light means a tire is well down, not that it has just started to drop, and the system is allowed twenty minutes to notice. A monthly check with a gauge catches the slow loss the standard was never designed to report.
Common questions
Should I inflate to the number on the tire sidewall?
No. The sidewall figure, often 44 to 51 psi, is the maximum pressure the tire is built to hold, not a recommendation. The number to use is on the placard inside the driver's door jamb, in the glovebox or on the fuel filler flap, and it is set by the vehicle manufacturer for that vehicle. Inflating to the sidewall maximum gives a harsh ride and wears the center of the tread.
Why does my pressure read higher after driving?
Driving heats the tire and heat raises pressure. A hot reading typically runs 4 to 6 psi above the cold value. Placard figures assume cold tires, meaning the vehicle has been parked for at least three hours or driven less than about a mile. Never bleed air out of a hot tire to hit the placard number, because it will be underinflated once it cools.
Which unit will my gauge be in?
It depends where the gauge and the vehicle come from. PSI is standard in North America, bar across most of Europe, kPa in Australia and in many owner's manuals, and kgf per square centimeter on older Asian gauges. One bar is 100 kPa exactly and about 14.5 psi, and kgf per square centimeter is close enough to bar that the two are often used interchangeably in practice.
Does cold weather change tire pressure?
Yes. Pressure moves roughly 1 psi for every 10 degrees Fahrenheit of ambient change, so a tire set correctly in summer can be several psi low by midwinter. This is why low pressure warnings cluster on the first cold morning of the season.
Related converters and guides
Sources
- Conversion factors from the NIST Guide to the SI. One psi is 6,894.757293168 pascals and one bar is exactly 100 kPa.
- 49 CFR 571.138, Standard No. 138. Source for the TPMS threshold: 25 percent below the placard pressure or the minimum in the standard's Table 1, whichever is higher, with the telltale lit within 20 minutes.
- The placard-versus-sidewall distinction, the 4 to 6 psi hot offset and the 1 psi per 10 °F rule are consistent across vehicle and tire manufacturer guidance. Figures here are the widely published values; your vehicle handbook takes precedence over any of them.