Tire Pressure Calculator

How temperature changes your tire pressure

Tire pressure is not a fixed number, it moves with the weather. Air contracts when it gets cold and expands when it warms, and the rule tire engineers use is simple: for every 10°F change in air temperature, tire pressure changes by about 1 PSI.

That sounds small until you run the math on a cold snap. If your tires were set to 32 PSI on a 70°F afternoon and the temperature drops to 20°F overnight, the pressure falls by (70 - 20) / 10 = 5 PSI, to about 27 PSI. Your tires did not leak; the air shrank.

The calculator above does two jobs. First, it converts between the three units you will see on gauges and stickers: PSI (pounds per square inch, the US standard), kPa (kilopascals, used on many imports and most digital gauges) and bar (used in Europe and on some motorcycle equipment). Second, it adjusts the cold pressure on your door sticker for the current or forecast temperature, so you know what to set.

Worked example: a 32 PSI sticker in a 20°F cold snap

Your driver's-door sticker says 32 PSI. You check the tires at 20°F and the gauge reads 28.2 PSI. Should you worry?

• Sticker pressure (cold): 32 PSI
• Sticker reference temperature: 68°F (the standard cold condition)
• Current temperature: 20°F
• Temperature difference: 20 - 68 = -48°F
• Pressure change: -48 / 10 = -4.8 PSI
• Expected reading now: 32 - 4.8 = 27.2 PSI

A gauge reading near 27 or 28 PSI is exactly right for that temperature, not a leak. But the part that matters is that you still want the tires at 32 PSI at whatever temperature you are driving in, so you would add about 4.8 PSI and inflate to roughly 37 PSI while it is still 20°F outside.

Now flip it. Set the same tires to 32 PSI on a 20°F morning and drive south into 80°F weather, and the pressure climbs by (80 - 20) / 10 = 6 PSI to about 38 PSI, six pounds over the sticker. That is the version that wears your tires unevenly and can trigger a warning light.

PSI, kPa and bar at a glance

Three units, one pressure. Here are the conversions a US driver runs into most:

The exact factors are 1 PSI = 6.895 kPa = 0.0689 bar. The NHTSA tire safety pages list the standard cold-inflation guidance and the load-rating tables behind those numbers.

Door sticker vs tire sidewall: which number to use

The number on the tire sidewall is the maximum cold pressure the tire can safely hold at its maximum load, not the pressure you should run. The number on the driver's-door jamb sticker is what the vehicle manufacturer calculated for your car's weight and handling. Use the door sticker.

Exceptions are real but rare: if you have changed to a different tire size than the factory spec, or you are towing or hauling a heavy load, the correct pressure changes. A light truck towing a trailer often needs its rear tires at the higher pressure listed on the sticker's heavy-load line, which can be 10 to 15 PSI above the everyday figure.

Two more rules worth knowing. Never set pressure by the maximum printed on the sidewall, because that is a ceiling rather than a target. And always measure when the tires are cold, meaning they have sat at least three hours or been driven less than a mile. Driving heats tires and can raise the reading 3 to 5 PSI, which is normal and expected.

How to check and set your tire pressure

The routine takes five minutes a month:

• Do it cold. First thing in the morning, before the car has moved.
• Check all four plus the spare. Tires lose about 1 PSI a month on their own, and a slow leak can hide for weeks.
• Use a good gauge. A $10 digital gauge is more accurate than the stick gauge in your glovebox.
• Adjust for the season. Add air in the fall as temperatures drop and let some out in late spring as they climb, which is exactly what this calculator estimates.
• Replace the valve caps. They keep dirt and moisture out of the valve core.

Under-inflated tires wear faster on the edges, hurt fuel economy and run hotter, which is a safety issue at highway speed. The US Department of Energy's fuel economy maintenance guide estimates that properly inflated tires can improve gas mileage by about 0.6% to 3%, on top of the tire life you save. For most drivers the temperature adjustment above is the difference between a tire that lasts 45,000 miles and one that lasts 35,000.

Towing, heavy loads and the second line on the sticker

Most door stickers have more than one row. Alongside the everyday cold tire pressure for a normal load there is usually a second line for a full load or towing, and it can be 5 to 15 PSI higher at the rear. If you are hauling a trailer, loading the bed of a truck or filling every seat for a road trip, that second line is the one to use.

Why it matters: a tire carries a fixed load at a given pressure. Add weight without adding air and the sidewall flexes more, the tire runs hotter, and the shoulders wear. A rear tire on a light truck might need 35 PSI solo and 44 PSI with a trailer hitched up, a 9 PSI difference that the everyday figure does not cover.

Three numbers to remember when you load up:

• Add 3 to 5 PSI at the rear for a heavy passenger load.
• Use the sticker's towing or heavy-load line exactly, not a guess.
• Never exceed the sidewall maximum, even when loaded.

After the trip, let the extra air back out. Over-inflated tires on an empty truck wear in the centre and ride harshly, which costs you the fuel economy you were trying to protect.

One more thing about the sticker: it is specific to the tire size the factory fitted. If you have changed to larger wheels or a different tire size, the factory pressure may no longer be right, and you should ask a tire shop for the correct figure for the new size and load. For most drivers, though, the door sticker plus the temperature adjustment in the calculator above is all you need.