What is my tire pressure?
Type the tire pressure on your gauge and the temperature when you read it, then a second temperature. The tire pressure calculator shows what the gauge will read then, or works back to the pressure to set.
- Tire pressure then
- 31.25 psi
A tire at 35 psi and 70 °F reads 31.25 psi at 30 °F.
- Change in pressure
- -3.75 psi
Tire pressure then: 31.25 psi. A tire at 35 psi and 70 °F reads 31.25 psi at 30 °F.
How to calculate
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.
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.
Formula: (P₁ + 1 atm) ÷ T₁ = (P₂ + 1 atm) ÷ T₂, with gauge pressures P and absolute temperatures T; P₂ = (P₁ + 1 atm) × T₂ ÷ T₁ − 1 atm.
- 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
Each example is checked against the calculator on every build.
- Tire pressure now 35 psi, Temperature now 70 °F, Temperature then 30 °F gives Tire pressure then 31.25 psi, Change in pressure -3.75 psi.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
- Tire pressure now 34.81 psi, Temperature now 68 °F, Temperature then 140 °F gives Tire pressure then 41.56 psi.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
- Tire pressure then 36 psi, Temperature now 68 °F, Temperature then 95 °F gives Tire pressure now 33.53 psi.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
- Tire pressure now 32 psi, Temperature now 75 °F, Tire pressure then 28 psi gives Temperature then 29.2 °F.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
How it works
Type three of the four values: the gauge pressure and temperature now (P₁, T₁) and then (P₂, T₂). The calculator finds the fourth.
A gauge shows the pressure above the air around it. The gas law needs absolute pressure, so the calculator adds Pₐ = 1 standard atmosphere = 101,325 Pa (14.696 psi). With temperatures in kelvins:
(P₁ + Pₐ) ÷ T₁ = (P₂ + Pₐ) ÷ T₂
- P₂ = (P₁ + Pₐ) × T₂ ÷ T₁ − Pₐ
- P₁ = (P₂ + Pₐ) × T₁ ÷ T₂ − Pₐ
- T₂ = T₁ × (P₂ + Pₐ) ÷ (P₁ + Pₐ)
- T₁ = T₂ × (P₁ + Pₐ) ÷ (P₂ + Pₐ)
Change in pressure = P₂ − P₁, shown in psi (2 decimal places) or kPa in Metric.
Every value is converted first: 1 psi = 4.4482216152605 N ÷ 0.00064516 m² ≈ 6,894.757 Pa, 1 kPa = 1,000 Pa, 1 bar = 100,000 Pa; K = (°F + 459.67) × 5/9 = °C + 273.15. The maths runs in floating point and rounds for display only.
Assumptions: the tire's volume and the amount of air in it stay the same (no leak, no added air), and the air outside is at 1 standard atmosphere. At high altitude the outside air is lower, so this slightly overstates the change.
Rules
- Gauge pressures are from 0 to 2,000 kPa (290 psi); temperatures from −100 °C to 200 °C (−148 °F to 392 °F). A typed value outside that shows a message on its field. A solved value outside it (for example a pressure below 0 on the gauge) gives no answer.
- With all four values typed, they must fit the equation; otherwise the page says the values do not agree.
Worked examples by hand
A cold morning. P₁ = 35 psi at T₁ = 70 °F = 529.67 °R, T₂ = 30 °F = 489.67 °R (the ratio of Rankine degrees equals the ratio of kelvins). P₂ = (35 + 14.696) × 489.67 ÷ 529.67 − 14.696 = 31.25 psi. Change = −3.75 psi.
After a drive. P₁ = 240 kPa at 20 °C = 293.15 K, T₂ = 60 °C = 333.15 K. P₂ = (240 + 101.325) × 333.15 ÷ 293.15 − 101.325 = 286.57 kPa.
The pressure to set. You want P₂ = 36 psi at 95 °F (554.67 °R) and fill at 68 °F (527.67 °R). P₁ = (36 + 14.696) × 527.67 ÷ 554.67 − 14.696 = 33.53 psi.
When the light comes on. Set to 32 psi at 75 °F (534.67 °R); it reads 28 psi at T₂ = 534.67 × (28 + 14.696) ÷ (32 + 14.696) = 488.87 °R = 29.2 °F.
Other questions people ask
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.