How the comparison works
The reading and both placard figures are converted to kilopascals, which allows the comparison to work whichever unit you chose. One psi is 6.894757 kilopascals and one bar is 100 kilopascals, so a reading of 28 psi is 1.93 bar or 193 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 reported in the unit you entered, together with the adjustment needed to reach the nearest end of the range.
Worked example
A gauge shows 28.0 psi and the placard gives 30 to 33 psi for the tyres fitted. The reading is 2.0 psi below the range, so the suggested adjustment is to add 2.0 psi. The same reading is 1.93 bar or 193 kPa.
These figures are illustrative. Use the pressure range printed for your own vehicle and the tyres currently fitted to it.
Where the correct pressure comes from
The correct pressure is a vehicle specification, not a tyre specification. It appears on a placard on the driver's door pillar, inside the fuel filler flap or in the handbook, and it often differs between front and rear tyres and between normal and fully laden conditions.
The Highway Code asks drivers to check tyres regularly and to keep them at the manufacturer's recommended pressure. A pressure that suits one car can be wrong for another of the same size, so a tyre-size table or a rule based on vehicle weight cannot replace the placard.
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 inside the tyre and raises the reading, so a warm tyre can be above the range without being over-inflated.
The placard often lists a higher figure for a full load. Choose the load condition that matches how the vehicle is being used, and use the same condition for the minimum and maximum you enter. A tyre pressure monitoring system warns about a loss of pressure; it is not a substitute for a gauge check.
Limits of this calculator
It compares numbers; it does not know your vehicle, your tyre size or the load you are carrying. It does not account for a faulty gauge, a slow puncture, valve leaks or the effect of altitude and ambient temperature.
For the effect of a different tyre size on gearing and the speedometer, use the tyre size calculator or the speedometer calculator.
For a change of tyre size and its effect on gearing, use the UK tyre size calculator.
For psi figures and US placard terminology, use the US tire pressure calculator.
Inputs for the tyre pressure calculator
Pressure unit
Choose the unit shown on your gauge. The result converts the reading to the other two units.
Measured tyre 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 pillar, fuel flap or handbook label for these tyres.
Highest placard pressure
Enter the upper figure for the same load condition, or repeat the lower figure when only one is given.
CALCULATOR QUESTIONS
Tyre pressure calculator UK questions
Does this tell me the right pressure for my car?
No. It compares your reading with the range you enter. The correct range comes from the vehicle placard, because it depends on the car, the tyres and the load.
Why does my reading rise after driving?
Friction warms the air inside the tyre and raises the pressure. Pressures are specified cold, so check before a long drive or after the vehicle has stood for a few hours.
Should I use the higher pressure for a full load?
Use the figure the placard gives for the load you are carrying. Enter that condition for both the minimum and the maximum so the comparison stays consistent.
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.