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Tires & Detailing · Free tool

Free Tire Pressure Calculator

Enter the pressure you set and the temperature then and now. The calculator shows what the tires hold today, how much air to add and the same figure in bar and kPa.

  • Free to use
  • No signup
  • Runs in your browser

Temperature change

PSI

From the placard on the driver’s door jamb, not the tire sidewall.

PSI
°F
°F

Unit converter

Unit

Expected pressure now

31.2 PSI

Down 3.8 PSI from a 40°F drop.

Add about 3.8 PSI to get back to the recommended 35 PSI.

Expected pressure in bar
2.15
Expected pressure in kPa
215
Change per 10°F here
0.94 PSI

Converted

PSI
35.0
Bar
2.41
kPa
241

A calculation is not a reading. Check the tires with a gauge when they are cold.

An estimate from the figures you enter, not a quote. Read the disclaimer. Updated .

02Your result

What your tire pressure result means

Air contracts as it cools. A tire that was right in September can be several PSI low in January without having lost any air.

  1. 01

    Expected pressure now

    What the tire should hold at today’s temperature if it has not leaked. It is a prediction, so confirm it with a gauge.

  2. 02

    Air to add

    The gap between that figure and the recommended cold pressure for your vehicle.

  3. 03

    PSI, bar and kPa

    The same pressure in the three units you will meet on gauges, door placards and air machines.

What to do next

03How it works

The tire pressure and temperature formula

For a fixed amount of air in a fixed space, pressure rises and falls in step with absolute temperature. That is Gay-Lussac’s law.

Pressure after a temperature change
P2 = (P1 + 14.7) ×T2 + 459.67T1 + 459.67− 14.7

P2 = (P1 + 14.7) × (T2 + 459.67) ÷ (T1 + 459.67) − 14.7

Adding 14.7 turns a gauge reading into absolute pressure. Adding 459.67 turns °F into an absolute temperature.

Unit conversion
1 PSI = 6.895 kPa = 0.0689 bar

kPa = PSI × 6.895. bar = PSI × 0.0689

The rule of thumb from tire makers: about 1 PSI for every 10°F.

What each term means

P1
The pressure you set, read on a gauge, in PSI.
T1
The air temperature when you set it, in °F.
T2
The air temperature now, in °F.
P2
The pressure the tire should hold now, in PSI.

What the tool assumes

  • The tire has not lost any air since you set it.
  • The tire is cold: it has matched the outside temperature and has not just been driven.
  • Sea-level atmospheric pressure of 14.7 PSI.
  • The tire’s volume does not change, which is close to true for a car tire.

04Worked example

Tire pressure example: a 40°F drop

Tires set to 35 PSI on a 70°F afternoon, checked again on a 30°F morning.

  1. 01

    Convert the pressure to absolute

    35 PSI + 14.7

    49.7 PSI

  2. 02

    Convert both temperatures

    70°F and 30°F, each + 459.67

    529.67 and 489.67

  3. 03

    Scale the pressure

    49.7 × 489.67 ÷ 529.67

    45.9 PSI

  4. 04

    Convert back to a gauge reading

    45.9 − 14.7

    31.2 PSI

  5. 05

    Find the shortfall

    35 − 31.2

    3.8 PSI

Pressure at 30°F

31.2 PSI

That is 3.8 PSI low, close to the rule of thumb of 1 PSI per 10°F, and equal to 2.15 bar or 215 kPa.

Try your own numbers

05Background

Why tire pressure changes with temperature

The air in a tire is a gas in a nearly fixed space, so its pressure follows its temperature. Michelin puts it simply: each 10°F drop in temperature equates to a loss of 1 PSI. Continental gives the same figure. The formula above reaches almost the same answer from physics.

NHTSA’s guidance on checking pressure is short and worth following exactly.

  • Check all tires, including the spare, at least once a month.
  • Check them cold, before driving or after the car has stood for a few hours.
  • Use the pressure on the Tire and Loading Information label on the driver’s door edge, or in the owner’s manual.
  • Do not use the number on the tire sidewall. That is the maximum the tire can hold, not the pressure your vehicle needs.

Do not wait for the dashboard light. Federal rules require the tire pressure monitoring system to warn you when a tire is 25 percent or more below the recommended pressure. On a car with a 35 PSI placard that is 26.3 PSI, so a tire can be 8 PSI low with the light still off. See what the TPMS light means.

Car instrument cluster at night showing the speedometer, a digital trip display and a red seat belt warning light

06When to use it

When to use a tire pressure calculator

Whenever the weather turns, and whenever a gauge or a manual uses a unit you do not.

  • At the first cold snap

    See how far the pressure has fallen before the warning light tells you.

  • Before a hot-weather trip

    Understand why a cold-set pressure reads higher after a long drive.

  • To convert units

    Turn a placard in kPa or bar into the PSI on your gauge.

  • When the TPMS light comes on

    Work out whether a temperature drop explains it or a tire is leaking.

Mistakes to avoid.

  • Inflating to the maximum pressure printed on the tire sidewall.
  • Checking pressure straight after a drive, when the tires are warm and read high.
  • Letting air out of warm tires to bring them down to the placard figure.
  • Blaming the weather for a tire that keeps losing pressure. A steady loss is a leak.

07Related guides and tools

Keep going from here.

Part of our Maintenance guide

Guides that go with this tool

Related tools

All free tools

08Questions

Tire Pressure Calculator: your questions.

How much does tire pressure change with temperature?

About 1 PSI for every 10°F, according to Michelin and Continental. Pressure falls as the air gets colder and rises as it warms.

What PSI should my tires be?

Use the pressure on the Tire and Loading Information label on the driver’s door edge, or in the owner’s manual. The number on the tire sidewall is the maximum the tire can hold, not the recommended pressure.

Why is my tire pressure light on when it is cold?

Cold air lowers the pressure in every tire. The warning system is required to light when a tire is 25 percent or more below the recommended pressure, and a sharp temperature drop can push a tire that was already a little low past that point.

How do I convert PSI to bar or kPa?

Multiply PSI by 0.0689 to get bar and by 6.895 to get kPa. 35 PSI is 2.41 bar or 241 kPa.

Should I check tire pressure hot or cold?

Cold. NHTSA advises checking tires at least once a month when they are cold, because driving warms the air inside and raises the reading.

09Estimates and sources

Read this before you rely on it.

Built and checked by CarTipsMentor Team. Last updated . How we check our work.

Sources

  1. [1]National Highway Traffic Safety Administration, Tires (TireWise)
  2. [2]Michelin, Michelin FAQs: tire pressure and temperature
  3. [3]Continental Tire, TPMS Warning Light: What It Means and What to Do
  4. [4]Code of Federal Regulations, 49 CFR 571.138: Tire pressure monitoring systems