How to use the tire pressure converter
- Enter a gauge pressure and choose its unit to see it in psi, bar, kPa and kgf/cm².
- To estimate the effect of temperature, enter the temperature when the pressure was measured and the new temperature.
- Press Calculate. Changing the unit converts the number in the box so it still means the same pressure.
Common pressures converted
| 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 |
| 33 | 2.28 | 228 | 2.32 |
| 35 | 2.41 | 241 | 2.46 |
| 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 |
| 50 | 3.45 | 345 | 3.52 |
| 80 | 5.52 | 552 | 5.62 |
How temperature changes tyre pressure
Air in a tyre behaves close to an ideal gas in a nearly fixed volume, so its absolute pressure is proportional to its absolute temperature. A tyre set to 35 psi at 20 °C would read about 31.6 psi at 0 °C. Set to 35 psi at 70 °F, it would read about 31.2 psi at 30 °F.
That matches the rule of thumb from Tire Rack of about 2% per 10 °F, or roughly 1 psi per 10 °F for car tyres. It’s why pressures set on a warm afternoon often read low the next cold morning, and why placard pressures are cold pressures.
Set pressures cold, to the vehicle placard. This estimate explains readings; it isn’t a recommendation to inflate above or below the vehicle maker’s figures, and it never replaces checking with a gauge.
Formulas used
1 psi = 6.894757 kPa 1 bar = 100 kPa 1 kgf/cm² = 98.0665 kPa
absolute pressure = gauge pressure + atmospheric pressure
T (K) = T (°C) + 273.15
P₂ (absolute) = P₁ (absolute) × T₂ (K) ÷ T₁ (K)
P₂ (gauge) = P₂ (absolute) − atmospheric pressure The model assumes the tyre’s volume stays constant. Real tyres stretch slightly with pressure and temperature, and air warmed by driving isn’t at the outside temperature, so treat the result as an estimate.
Frequently asked questions
What pressure should my tyres be?
Use the cold pressures on your vehicle’s placard (usually on the driver’s door jamb, fuel flap or glovebox) or in the owner’s manual. The maximum pressure moulded on the sidewall is the tyre’s limit, not the recommended pressure. This tool converts and estimates; it doesn’t recommend a pressure.
How much does tyre pressure change with temperature?
Roughly 2% for every 10 °F (about 5.6 °C), which for typical car tyres is around 1 psi per 10 °F. The calculator works it out exactly from gas laws for your starting pressure and temperatures.
Should I let air out when tyres are hot?
Generally not. Recommended pressures are cold pressures, and the rise from driving is expected. Checking and adjusting when the tyres are cold, before driving more than a few miles, gives a reading you can compare with the placard.
Why does the calculation need atmospheric pressure?
A gauge reads the difference between the air in the tyre and the air outside. Gas laws work on absolute pressure, so the calculator adds atmospheric pressure, scales by absolute temperature, then subtracts it again. Standard sea-level pressure (101.325 kPa) is the default; at high altitude, a lower value is more accurate.
Is bar the same as kgf/cm²?
Very nearly but not exactly. 1 kgf/cm² is 98.0665 kPa and 1 bar is 100 kPa, a difference of about 2%. Some older gauges and placards use kgf/cm².
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