Tyre Pressure and Temperature Calculator

Pressure now, bar

Temperature now, °C

Temperature later, °C

Result

Becomes, bar1.76
In psi25.5
Change, bar-0.44
Per 10 °C, bar0.11
Temperature change, °C-40
What to doPressure has dropped — top the tyres up cold, before driving.

Why the warning light comes on in autumn

Works out what tyre pressure becomes at a different air temperature, using Gay-Lussac's law with the atmosphere properly accounted for.

Air in a closed volume follows a simple law: pressure is proportional to absolute temperature. When it gets colder outside the molecules slow down, press on the walls less, and the gauge reads lower — with not a gram of air having escaped.

The trap is that a gauge reads pressure above atmospheric while the law works on absolute pressure. Done properly the sum needs the atmosphere added in and the temperatures in kelvin, otherwise the correction comes out far too large.

What this calculator shows

The pressure at the new temperature, in bar and in psi
The change per ten degrees — the familiar rule of thumb
Whether to top the tyres up or leave them alone

What to keep in mind

The tyre is assumed airtight: real leakage through the valve and the rubber adds about 0.05 bar a month.
Atmospheric pressure is taken as 1.013 bar; at altitude the correction is slightly larger.

FAQs

How much pressure do tyres lose in the cold?

About 0.1 bar for every 10 degrees of cooling. From a summer +20 to a winter −20 that is roughly 0.4 bar, nearly a fifth of the specification.

When should I measure?

On cold tyres, before driving. Ten kilometres warms the rubber and adds 0.2 to 0.3 bar, which is normal — do not bleed it off.

Should I run higher pressure in winter?

Not above the manufacturer's figure, and certainly not below it. That figure is quoted cold, so winter simply means checking it more often.

Does nitrogen help?

Against temperature swings, barely: nitrogen obeys the same law. There is a small benefit from the absence of moisture, but in everyday use it is not noticeable.

Worked example

From a warm September to a frozen November

Input: 2.2 bar at +20 °C, now −20 °C

Output: 1.76 bar — a drop of 0.44

Note: This is precisely when the dashboard warning appears. Half a bar under the specification is genuinely unsafe: the tyre runs hot, the sidewall flexes, and fuel consumption climbs.

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Tyre Pressure and Temperature Calculator

Result

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