How the comparison works
The reading and both placard figures are converted to kilopascals so the comparison works in whichever unit you chose. One psi is 6.894757 kilopascals and one bar is 100 kilopascals, so a reading of 30 psi is 2.07 bar or 207 kPa.
The verdict is below the range when the reading is lower than the minimum, above it when higher than the maximum, and within it otherwise. The distance from the range is shown in the unit you entered, with the adjustment needed to reach the nearest end.
Worked example
A gauge shows 30.0 psi and the placard gives 32 to 35 psi for these tires. The reading is 2.0 psi below the range, so the suggested adjustment is to add 2.0 psi. The same reading is 2.07 bar or 207 kPa.
These figures are illustrative. Use the range printed for your own vehicle and the tires fitted to it.
Where the correct pressure comes from
In the United States the correct pressure is the vehicle manufacturer's specification, printed on the tire placard fixed to the driver's door jamb or the edge of the door. It is not the maximum pressure moulded into the tire sidewall, which is a limit for the tire rather than a recommendation for the car.
The placard usually gives a front and rear figure and sometimes a higher figure for a fully loaded vehicle. Because the specification depends on the vehicle, a tire size table or a rule based on vehicle weight cannot replace it.
Cold readings, load and temperature
Pressures are specified cold, meaning before the vehicle has been driven for more than a short distance. Driving warms the air and raises the reading, so a warm tire can be above the range without being over-inflated.
Choose the load condition that matches how the vehicle is being used and enter that condition for both the minimum and the maximum. A tire pressure monitoring system warns about a loss of pressure, but it does not replace a gauge check.
Limits of this calculator
It compares numbers; it does not know your vehicle, your tire size or your load. It does not account for a faulty gauge, a slow leak, a damaged valve or the effect of ambient temperature and altitude.
For the effect of a different tire size on gearing and the speedometer, use the tire size calculator or the speedometer calculator.
For a change of tire size and its effect on gearing, use the US tire size calculator.
For bar figures and UK placard terminology, use the UK tyre pressure calculator.
Inputs for the tire pressure calculator
Pressure unit
Choose the unit shown on your gauge. The result converts the reading to the other two units.
Measured tire pressure
Enter the cold reading from your gauge, taken before the vehicle has been driven far.
Lowest placard pressure
Enter the lower figure from the door jamb placard for the tires currently fitted.
Highest placard pressure
Enter the upper figure for the same load condition, or repeat the lower figure when only one is given.
CALCULATOR QUESTIONS
Tire pressure calculator US questions
Should I inflate to the number on the tire sidewall?
No. The sidewall figure is the tire's maximum, not the vehicle's recommendation. Use the door placard for normal driving.
Why does the reading rise after driving?
Friction warms the air inside the tire and raises the pressure. Pressures are specified cold, so check before a long drive.
Why are front and rear figures different?
The manufacturer sets pressure by axle load and vehicle design. Enter the figure for the axle you are checking, or check each axle separately.
What if the placard gives only one pressure?
Enter the same value as both the lowest and the highest. The calculator then reports whether the reading matches that single specification.