Cold vs Hot Tire Pressure Calculator
Air in a closed tire follows temperature. Check cold. Do not bleed a hot tire down to the door sticker.
What it does
Converts a gauge reading at one ambient temperature to the gauge reading at another, using P_abs / T_abs with atmosphere 14.696 psi and Rankine (°F + 459.67).
How to use it
Enter the cold (or current) gauge PSI and the two Fahrenheit temperatures. Morning vs afternoon is the usual pair.
How to read the result
A drop overnight can turn on TPMS without a leak. The ~1 psi per 10 °F rule of thumb is close around typical passenger pressures; the exact law uses absolute pressure, so we show both. This is a parked tire vs ambient air. Extra rise from highway heat is not a number we will publish — it is not a constant.
What this does not calculate
Constant-volume ideal gas, standard atmosphere, no altitude correction. Does not replace a leak test.
Does not recommend a set pressure. Use the door placard. Does not model TPMS thresholds.
Questions
- Why absolute pressure?
- Gay-Lussac is P/T with P above vacuum, not above atmosphere. Adding 14.696 psi is the honest version. Skipping it understates the change.
- Should I set pressure when the tires are hot?
- No. NHTSA and USTMA: parked three hours, or driven under a mile. Heat raises the reading.
Related tools
- PSI to kPa to Bar ConverterThe sidewall number is a maximum. The door sticker is what the car wants. Convert units here — do not mix them up.
- Tire Wear Pattern GuideThe tread is a clue. It is not a work order. Pick the pattern you see; we tell you what to check.
- Tire Load Capacity vs Axle WeightWill these tires hold that axle? We add the pounds from the load-index table. We do not invent an air pressure.
Estimate — not a fitment approval.
