About the Tire Pressure Temperature Calculator
This tire pressure temperature calculator predicts how your tire pressure will change when the temperature changes. Enter the pressure you set (as read on a normal gauge), the air temperature when you set it, and a new temperature — for example a cold morning after a warm afternoon fill-up — and it calculates the new gauge pressure, the change in PSI and the equivalent in kPa and bar.
It explains why the tire-pressure warning light often comes on during the first cold snap of autumn, and helps you decide how much extra pressure to add when you inflate in a warm garage before driving in freezing weather. It also works the other way, showing how much pressure builds up as tires warm in summer heat.
The calculation uses Gay-Lussac’s law on absolute pressure (gauge pressure plus atmospheric pressure) and assumes the tire volume stays the same and no air leaks out. Always set your tires to the pressure on the driver’s door-jamb placard when they are cold — before driving or after the car has been parked for a few hours.
With the default inputs, the new tire pressure is 30.3 psi. Change any value above to recalculate instantly.
How to use the tire pressure temperature calculator
- 1Choose Fahrenheit or Celsius.
- 2Enter the pressure you set on the gauge and the air temperature at the time.
- 3Enter the new temperature you expect (e.g. tomorrow morning’s low).
- 4Read the new pressure and how much it changed.
- 5Use the “set now” figure if you are inflating in a warm place before cold weather.
Formula and method
At a constant volume, the absolute pressure of a gas is proportional to its absolute temperature (Gay-Lussac’s law). Tire gauges show gauge pressure, which is absolute pressure minus atmospheric pressure, so the calculator adds 14.696 psi (one standard atmosphere), scales by the ratio of kelvin temperatures and subtracts the atmosphere again.
Because the atmospheric part does not change, a typical 30–35 psi car tire moves by about 1 psi for every 10°F (roughly 2 psi per 10°C). The “set now” figure reverses the calculation to show how much to inflate at the current temperature so the tire reads your target once it reaches the new temperature. Volume changes and altitude are ignored.
- P₁
- Gauge pressure when set
- P₂
- Gauge pressure at the new temperature
- T₁, T₂
- Air temperatures in kelvin
- Pₐₜₘ
- Atmospheric pressure, 14.696 psi at sea level
Worked examples
35 psi set at 70°F, overnight low of 20°F
70°F is 294.26 K and 20°F is 266.48 K. The absolute pressure of 49.70 psi scales by 266.48 ÷ 294.26 to 45.00 psi, which is 30.3 psi on the gauge — a drop of about 4.7 psi, or just under 1 psi per 10°F.
2.4 bar (34.8 psi) set at 20°C, morning at −5°C
20°C is 293.15 K and −5°C is 268.15 K. Absolute pressure falls from 49.50 to 45.27 psi, so the gauge reads about 30.6 psi (2.11 bar) — a drop of 4.2 psi.
Summer heat: 33 psi set at 65°F, tires sitting at 105°F
Warming from 65°F to 105°F raises absolute pressure from 47.70 to 51.33 psi, so the gauge reads about 36.6 psi. That is normal — never let air out of hot tires to reach the cold placard figure.
Frequently asked questions
How much does tire pressure drop per degree?+
For typical passenger-car tires around 30–35 psi, pressure changes by roughly 1 psi for every 10°F, or about 0.1 psi per °F (around 2 psi per 10°C). Higher-pressure tires change slightly more because the effect is proportional to absolute pressure.
Why does my tire pressure light come on when it gets cold?+
Cold air shrinks the gas in the tire, lowering pressure. A 40–50°F overnight drop can remove 4–5 psi, which can push a tire that was already a few psi low below the TPMS warning threshold (25% under the placard pressure on US vehicles) even though nothing is leaking. Top up to the placard pressure when the tires are cold.
Should I add air when it gets cold?+
Yes. Recommended pressures are cold pressures, so when outdoor temperatures fall you should re-inflate to the figure on the door-jamb placard, checked before driving. If you inflate in a heated garage, add a little extra so the pressure is right outside.
Should I let air out of hot tires?+
No. Pressure rises by several psi as tires warm up from driving or sun, and that rise is accounted for in the recommended cold pressure. Letting air out while hot leaves the tire under-inflated once it cools.
Does nitrogen stop pressure changing with temperature?+
No. Nitrogen follows the same gas law as air (which is already about 78% nitrogen), so it changes pressure with temperature by essentially the same amount. Its main claimed benefit is slower leakage through the rubber.