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Tire Pressure Calculator

AutomotiveTyres and Wheels
Units

Gauge reading, as shown on a tire gauge.

Pressure at the new temperature —
Change
—
Sensitivity
—
Absolute pressure after
—

Tire pressure changes with temperature because the air inside is a fixed volume of gas. Add atmospheric pressure to the gauge reading, scale it by the ratio of absolute temperatures, then subtract atmosphere again. A tire at 35 psi on a 70 °F day reads about 31.2 psi at 30 °F, a drop of 3.8 psi.

About this tool

Drivers notice it every autumn: the low-pressure light comes on during the first cold morning even though nothing leaked. This calculator predicts the gauge reading a tire will show after the surrounding air warms or cools, given the pressure you measured and the temperature at the time. It works in psi with Fahrenheit or in kPa with Celsius, shows the change, how many psi or kPa move per 10 degrees for your tire, and the full working. Use it to decide how much to add before a cold spell, or to understand why a reading taken in a heated garage differs from one taken outside. It assumes the tire volume stays constant and no air escapes, and it treats the temperature of the air inside the tire as equal to the outside air, which is only true once the tire has sat for a few hours.

How to use it

  1. Pick your units

    Choose psi and °F or kPa and °C; the fields switch to match.

  2. Enter the measured pressure

    Type the gauge reading and the air temperature when you took it.

  3. Enter the new temperature

    Give the temperature you expect, such as tomorrow morning's forecast low.

  4. Read the result

    The new pressure, the change and the per-10-degree sensitivity appear with the working beneath.

Examples

35 psi at 70 °F, then 30 °F

Units
psi and °F
Pressure you measured
35
Air temperature when measured
70
New temperature
30

Result Pressure at the new temperature: 31.2
Change: -3.8 psi
Sensitivity: 0.94 psi per 10 °F
Absolute pressure after: 45.9 psi

  1. Absolute pressure before = 35 + 14.696 = 49.696 psi
  2. Absolute temperatures: 70 + 459.67 = 529.67, 30 + 459.67 = 489.67 °R
  3. Absolute pressure after = 49.696 × 489.67 ÷ 529.67 = 45.943 psi
  4. Gauge pressure after = 45.943 − 14.696 = 31.2 psi

A 40 °F overnight drop takes almost 4 psi out of a tire set in a warm garage, close to the 1 psi per 10 °F rule of thumb.

240 kPa at 20 °C, then −10 °C

Units
kPa and °C
Pressure you measured
240
Air temperature when measured
20
New temperature
-10

Result Pressure at the new temperature: 205.1
Change: -34.9 kPa
Sensitivity: 11.64 kPa per 10 °C
Absolute pressure after: 306.4 kPa

  1. Absolute pressure before = 240 + 101.325 = 341.325 kPa
  2. Absolute temperatures: 20 + 273.15 = 293.15, -10 + 273.15 = 263.15 K
  3. Absolute pressure after = 341.325 × 263.15 ÷ 293.15 = 306.395 kPa
  4. Gauge pressure after = 306.395 − 101.325 = 205.1 kPa

A tire inflated to 2.4 bar indoors reads about 2.05 bar on a frosty morning, a loss of roughly 35 kPa.

32 psi at 60 °F, then 100 °F

Units
psi and °F
Pressure you measured
32
Air temperature when measured
60
New temperature
100

Result Pressure at the new temperature: 35.6
Change: +3.6 psi
Sensitivity: 0.9 psi per 10 °F
Absolute pressure after: 50.3 psi

  1. Absolute pressure before = 32 + 14.696 = 46.696 psi
  2. Absolute temperatures: 60 + 459.67 = 519.67, 100 + 459.67 = 559.67 °R
  3. Absolute pressure after = 46.696 × 559.67 ÷ 519.67 = 50.29 psi
  4. Gauge pressure after = 50.29 − 14.696 = 35.6 psi

Heat raises the reading by about 3.6 psi, so a tire checked hot looks over-inflated when it is not.

How it is calculated

P2 = (P1 + Patm) × T2 ÷ T1 − Patm

P1
gauge pressure measured at temperature T1
P2
gauge pressure at temperature T2
Patm
atmospheric pressure, 14.696 psi or 101.325 kPa
T1, T2
absolute temperatures: °F + 459.67 (rankine) or °C + 273.15 (kelvin)

A tire gauge reads pressure above the atmosphere, but gas laws work on absolute pressure, so atmospheric pressure is added first. With volume and the amount of air fixed, absolute pressure is proportional to absolute temperature (Gay-Lussac's law). Scaling by the temperature ratio and subtracting the atmosphere again gives the new gauge reading. For a typical car tire near 35 psi this works out to roughly 0.9 to 1 psi per 10 °F, which is where the familiar rule of thumb comes from.

When not to use it

  • Do not use it to correct a reading taken right after driving; tire heat is not the air temperature.
  • It cannot find a leak, since it assumes no air is lost.
  • It does not replace the placard pressure on the driver's door jamb.

Common mistakes

  • Applying the temperature ratio to the gauge reading instead of the absolute pressure.
  • Using Celsius or Fahrenheit directly in the ratio rather than kelvin or rankine.
  • Setting pressure in a warm garage and expecting the same reading outside on a frosty morning.
  • Inflating hot tires down to the placard value, which leaves them low once cool.

Frequently asked questions

How much does tire pressure drop per 10 degrees?

For a passenger tire around 30 to 35 psi, about 0.9 to 1 psi for every 10 °F, or roughly 6 to 7 kPa for every 5 °C. The exact figure depends on the starting pressure, which is why the calculator shows the sensitivity for your own numbers.

Why add atmospheric pressure before scaling?

The gas law relates absolute pressure to absolute temperature. A gauge reading of zero still means 14.7 psi of air inside the tire, so ignoring the atmosphere would understate the change by about 30 percent.

Should tires be set to the placard pressure when cold or warm?

Vehicle makers specify cold inflation pressure, meaning the tire has not been driven for at least three hours or more than about a mile. Readings taken after a drive are higher and should not be bled down.

Why did my tire pressure light come on on a cold morning?

US monitoring systems must warn when a tire falls 25 percent or more below the placard pressure. A tire already a few psi low in mild weather can cross that line after a sharp overnight drop, even with no puncture.

Does nitrogen inflation stop pressure changing with temperature?

No. Nitrogen follows the same gas law as air, which is already about 78 percent nitrogen. Its benefit, if any, is slower seepage through the rubber, not a different response to temperature.

How much air should I add before winter?

Enter the current reading and temperature, then the expected low. The difference shown in the change line is roughly what you will lose, so many drivers top up to the placard value on the coldest mornings rather than on warm afternoons.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Tire Pressure Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/automotive/tyres-and-wheels/tire-pressure-calculator/

MLA

Tools Directory Editorial Team. “Tire Pressure Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/automotive/tyres-and-wheels/tire-pressure-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Tire Pressure Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/automotive/tyres-and-wheels/tire-pressure-calculator/.

Link<a href="https://tools.milunir.com/automotive/tyres-and-wheels/tire-pressure-calculator/">Tire Pressure Calculator – Tools Directory</a>