How it works
Air in a tire behaves like an ideal gas, so its absolute pressure rises and falls in step with absolute temperature (Gay-Lussac’s law). A tire gauge reads pressure above the atmosphere, so the calculator adds atmospheric pressure, scales by the temperature ratio in kelvins or degrees Rankine, then subtracts it again.
For a typical car tire near 35 psi, that works out to about 1 psi for every 10°F, which is why the low-pressure warning often comes on during the first cold mornings of fall. The air has not leaked out; it has contracted.
The same effect works in reverse while driving. Tires warm up from flexing and can read 2 to 4 psi higher after a drive, which is why the recommended pressure on the door sticker is a cold pressure, measured before driving or after the car has sat for a few hours.
Formula
P₂ = (P₁ + Pₐₜₘ) × T₂ ÷ T₁ − Pₐₜₘ, with T in kelvins (°C + 273.15) or degrees Rankine (°F + 459.67) and Pₐₜₘ = 14.696 psi or 101.325 kPa.
Questions people ask
How much does tire pressure drop in cold weather?
About 1 psi for every 10°F drop in temperature. A tire set to 35 psi on a 70°F afternoon reads about 30 psi on a 20°F morning.
Should I add air when it gets cold?
Yes. Check the tires cold and fill them to the pressure on the driver’s door jamb sticker for the temperature you are driving in. Underinflated tires wear faster, use more fuel and handle worse.
Should I let air out when tires are hot?
No. Pressure rises a few psi as tires warm up on the road, and the recommended pressure already allows for that. Letting air out of hot tires leaves them low once they cool.
Does this work for nitrogen-filled tires?
Yes. Nitrogen follows the same gas law as air, which is already about 78% nitrogen, so its pressure changes with temperature by almost exactly the same amount.