How to use the tire pressure and temperature calculator
- Enter the gauge pressure you measured and choose its unit: psi, kPa or bar.
- Enter the temperature when measured and the temperature to estimate for, and choose Fahrenheit or Celsius.
- Optionally enter the outside air pressure. Leave it blank to use standard sea-level pressure.
- Read the estimated gauge pressure, the change and the change for each 10 degrees.
Formula
absolute pressure = gauge pressure + outside air pressure
P2 = P1 x T2 / T1, with T in kelvin
estimated gauge pressure = P2 - outside air pressure
Kelvin = Celsius + 273.15, and Celsius = (Fahrenheit - 32) x 5 / 9. Standard sea-level air pressure is 14.696 psi, 101.325 kPa or 1.01325 bar.
A tire set at 70 degrees F and checked at 20 degrees F
A tire reads 32 psi at 70°F (294.26 K). The absolute pressure is 32 + 14.696 = 46.70 psi. At 20°F (266.48 K) it becomes 46.70 x 266.48 / 294.26 = 42.29 psi absolute, which is 27.59 psi on a gauge. The tire drops 4.41 psi, about 0.88 psi for every 10 degrees.
A tire at 220 kPa gauge at 15°C rises to 242.3 kPa at 35°C. At a high elevation where the outside air pressure is 11.8 psi, the same 32 psi tire at 70°F falls to 28.69 psi at 30°F.
Why pressure follows temperature
Air in a sealed tire is a fixed amount in a nearly fixed space. When the air gets colder its pressure falls, and when it gets warmer the pressure rises, in proportion to the absolute temperature. A common rule of thumb is that pressure changes by about 1 psi for every 10 degrees Fahrenheit. The page calculates it from the gas relation, so the numbers are close to that rule.
The relation uses absolute pressure, not the number on the gauge. A gauge reads the pressure above the outside air, so the outside air pressure must be added first and subtracted again at the end. That makes a small difference at sea level and a larger one in the mountains.
Using the estimate
Driving heats the tires, so check pressure when they are cold, and use the pressure on the vehicle’s placard, not the number molded into the tire. The estimate shows the trend and does not replace a gauge. Tires also leak slowly, and moisture or a different gas can change the result. See also the Tire Size Calculator, the Tire Life Calculator and the Stopping Distance Calculator.
Assumptions and limits
- The amount of air and the volume of the tire stay fixed.
- The air temperature inside the tire equals the temperature you enter.
- Standard sea-level pressure is used for the outside air unless you enter another.
- Tires flex and warm up in use, which is not modeled.
- The pressure and temperatures shown when the page opens are examples.
Frequently asked questions
How much does pressure change for each degree?
In the example, 0.88 psi per 10 degrees Fahrenheit at 32 psi. The exact change depends on the starting pressure and temperature, which is why the page calculates it.
Why do I need the outside air pressure?
A gauge shows pressure above the outside air. The gas relation needs the full pressure, so the outside air is added before the change and taken away afterward.
Can I use this to set pressure for a trip?
You can use it to see how much a temperature swing may change a reading. Always set pressure from the placard on the vehicle and check it with a gauge when the tires are cold.
What happens at very low temperatures?
The page accepts temperatures from minus 100 to 250 degrees Fahrenheit. If the estimate would fall to zero gauge pressure or below, it says the model no longer applies.