Silca Tire Pressure Calculator

From the placard in the driver’s door opening. Leave at 0 to skip the top-up figure.
Ambient pressure falls with altitude, so the same air in the tyre reads higher on a gauge.
Pressure now (gauge, in your unit)
31.45
Tire Pressure Calculator
Pressure now
31.45 psi
Pressure now
216.9 kPa
Pressure now
2.169 bar
Change from the set pressure
-3.55
Absolute pressure when set
342,642 Pa
Absolute pressure now
318,180 Pa
Sensitivity around this pressure
0.939 psi/10degF
Ambient pressure at the checking altitude
101,325 Pa
Worked out from Gay-Lussac’s law at constant volume: P/T is constant for absolute pressure and absolute temperature and 3 other sources.
Set and check pressures cold: driving heats a tyre and raises its pressure by several psi.
The correct cold inflation pressure for a car, van or trailer is the vehicle manufacturer’s figure on the tyre information placard in the driver’s door opening (and in the owner’s manual), never the maximum pressure moulded on the sidewall. This tool only corrects a pressure you supply for temperature and altitude; it does not choose a pressure for you, and it does not model bicycle tyre pressure by rider weight.

How this is worked out

5 steps
  1. Absolute pressure when set = 35 psi gauge + 14.696 psi ambient = 49.696 psi.
  2. Gas law at constant volume: absolute pressure scales with absolute temperature, 273.15 K / 294.15 K = 0.928608.
  3. Absolute pressure now = 46.148 psi; less 14.696 psi ambient gives 31.45 psi on the gauge.
  4. Around this pressure, 10 degrees F is worth 0.939 psi - the origin of the "1 psi per 10 F" rule.
  5. No placard pressure given, so no top-up figure is reported.

Results are general information, not financial, tax, legal, medical or engineering advice.