# What is my tire pressure?

Works out a tire’s pressure at a new temperature from its gauge pressure at another temperature (Gay-Lussac’s law with absolute pressure), or the cold pressure to set, in psi, kPa or bar.

- Page: https://www.acalculator.org/everyday-life/tire-pressure-calculator
- JSON spec: https://www.acalculator.org/everyday-life/tire-pressure-calculator.json
- Version: a0e23d3fbc50

## Default answer

Example with the default inputs (Tire pressure now 35 psi, Temperature now 70 °F, Temperature then 30 °F): A tire at 35 psi and 70 °F reads 31.25 psi at 30 °F.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| p1 | Tire pressure now | The gauge reading at the first temperature. |
| t1 | Temperature now | The temperature of the tire (or the air around a parked tire) at that reading. |
| p2 | Tire pressure then | The gauge reading at the second temperature. |
| t2 | Temperature then | The second temperature of the tire. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| p1 | Tire pressure now | The gauge reading at the first temperature. |
| t1 | Temperature now | The temperature of the tire (or the air around a parked tire) at that reading. |
| p2 | Tire pressure then | The gauge reading at the second temperature. |
| t2 | Temperature then | The second temperature of the tire. |
| change | Change in pressure | The second gauge reading minus the first. |

## Method

(P₁ + 1 atm) ÷ T₁ = (P₂ + 1 atm) ÷ T₂, with gauge pressures P and absolute temperatures T; P₂ = (P₁ + 1 atm) × T₂ ÷ T₁ − 1 atm.

## Assumptions

- The tire’s volume and the amount of air in it stay the same; no air leaks out or is added.
- The air around the tire is at 1 standard atmosphere (101.325 kPa, 14.696 psi), which a gauge does not count.
- Temperatures convert to kelvins: (°F + 459.67) × 5/9, °C + 273.15.

## Worked examples

1. p1 = 241,316.505261, t1 = 294.261111, t2 = 272.038889 gives p2 = 215,440.657638, change = -25,875.847623. Source: OpenStax, Chemistry 2e, §9.2 (Amontons’s or Gay-Lussac’s law: P₁/T₁ = P₂/T₂ at constant volume), https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law; NIST SP 811 Appendix B.9 (1 atm = 101,325 Pa; 1 psi = 6,894.757 Pa), https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9.
2. p1 = 240,000, t1 = 293.15, t2 = 333.15 gives p2 = 286,573.426573. Source: OpenStax, Chemistry 2e, §9.2 (Amontons’s or Gay-Lussac’s law: P₁/T₁ = P₂/T₂ at constant volume), https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law.
3. p2 = 248,211.262554, t1 = 293.15, t2 = 308.15 gives p1 = 231,196.678948. Source: OpenStax, Chemistry 2e, §9.2 (Amontons’s or Gay-Lussac’s law: P₁/T₁ = P₂/T₂ at constant volume), https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law; NIST SP 811 Appendix B.9 (1 atm = 101,325 Pa; 1 psi = 6,894.757 Pa), https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9.
4. p1 = 220,632.233381, t1 = 297.038889, p2 = 193,053.204209 gives t2 = 271.594378. Source: OpenStax, Chemistry 2e, §9.2 (Amontons’s or Gay-Lussac’s law: P₁/T₁ = P₂/T₂ at constant volume), https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law; NIST SP 811 Appendix B.9 (1 atm = 101,325 Pa; 1 psi = 6,894.757 Pa), https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9.

## FAQ

### How much does tire pressure drop when it gets cold?

About 1 psi for every 10 °F at normal car tire pressures. A tire set to 35 psi at 70 °F reads about 31.2 psi at 30 °F, a drop of 3.75 psi for 40 °F.

### Why does temperature change tire pressure?

The air in a tire is a gas at a nearly fixed volume. Its absolute pressure rises and falls in step with its absolute temperature (Gay-Lussac’s law), so warm air pushes harder on the tire and cold air less.

### Why does the calculator add 14.7 psi?

A tire gauge shows the pressure above the air around you, not the absolute pressure. The gas law needs absolute pressure, so the calculator adds 1 atmosphere (14.696 psi, 101.325 kPa), applies the temperature ratio, then takes it off again.

### Should I check tire pressure hot or cold?

Cold. The pressure on the door sticker is a cold pressure, read before driving or after the car has been parked for a few hours. Driving warms the tires and raises the reading.

### What pressure should I set in a warm garage for a cold day?

Pick the pressure you want at the cold temperature and solve for the pressure now. To read 36 psi at 95 °F when you fill at 68 °F, set about 33.5 psi; the other way round, set a little more in a warm garage.

### Does this work for bike tires?

Yes, the gas law is the same. At higher pressures each degree changes the pressure more: a road tire at 90 psi changes by about 2 psi for every 10 °F.

## Sources

- OpenStax, Chemistry 2e, section 9.2: at constant volume and amount, the pressure of a gas is proportional to its absolute temperature (Amontons’s or Gay-Lussac’s law, P₁/T₁ = P₂/T₂). CC BY 4.0, retrieved 2026-10-02. https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law
- NIST Special Publication 811, Appendix B.9: 1 standard atmosphere = 101,325 Pa exactly; 1 psi = 6,894.757 Pa. Retrieved 2026-10-02. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9