Enrichment · Passion Projects · Cars and travel

Same air, colder morning,
lower reading.

Type the gauge reading and the temperature it was taken at. The calculator applies the gas law to tell you what the tire will read on a cold morning or a hot afternoon, and how that compares with the placard.

Adjust for temperatureWhat you get

The tire pressure calculator

The worked example is made up: a tire set to 35 psi in a 70 °F garage before a 20 °F morning. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
The gauge reading at another temperature, the change, the rule-of-thumb figure, the percent from the placard and a table across temperatures. It prints as a summary and downloads as a spreadsheet.
Who it is for
New drivers and families, physics and chemistry students studying gas laws, and cyclists.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard, in psi or kPa and °F or °C. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose the units.
  2. Type the gauge reading and the air temperature at the time.
  3. Type the other temperature.
  4. Add the placard pressure from the door sticker.
  5. Read the new pressure and add air if it falls short.

Adjust for temperature

How the pressure changes

Gay-Lussac’s law, on absolute scales.

The worked example: a cold snapGauge + air49.70 psi35 + 14.696 absolute× T₂ ÷ T₁× 0.9056479.67 ÷ 529.67 rankineAbsolute at 20 °F45.00 psisame air, colderGauge at 20 °F30.3 psi-4.7 psiThe worked example: a cold snapGauge + air49.70 psi35 + 14.696 absolute× T₂ ÷ T₁× 0.9056479.67 ÷ 529.67 rankineAbsolute at 20 °F45.00 psisame air, colderGauge at 20 °F30.3 psi-4.7 psi
The gas law needs absolute pressure and absolute temperature. Gauge readings leave out the atmosphere, and Fahrenheit starts at the wrong zero, so both are shifted first.

The gas law

For a fixed amount of gas in a fixed volume, pressure is proportional to absolute temperature. Two corrections come first: a tire gauge reads pressure above the atmosphere, so 14.696 psi is added; and temperature must be measured from absolute zero, so 459.67 is added to °F (or 273.15 to °C). A tire’s volume changes a little, so real tires follow the law closely but not exactly.

What to do with it

Set tires to the pressure on the placard inside the driver’s door when they are cold. A garage-filled tire can be several psi low on a winter morning, which is why low-pressure warning lights come on in the first cold snap. Tires warm up by driving, so a hot reading above the placard is normal; letting air out then leaves them low once they cool.

Sources

  1. National Highway Traffic Safety Administration. Tires: tire pressure, placard and checking cold tires (nhtsa.gov).
  2. OpenStax. Chemistry 2e (2019), section 9.2, Relating pressure, volume, amount and temperature: the ideal gas law (openstax.org).
  3. National Institute of Standards and Technology. The standard atmosphere, 101.325 kPa, and temperature scales (nist.gov).
  4. U.S. Department of Energy. FuelEconomy.gov: keeping tires properly inflated (fueleconomy.gov).

The worked example is made up. Your vehicle’s placard and manual take precedence. Your figures stay in this browser. Support: team@caveatacademic.com.