Tyre Pressure Calculator
Calculate the correct tyre pressure when changing tyre sizes. Our load-matching calculator finds the pressure your new tyres need to carry the same weight as your original tyres.
This calculator provides reference values only. Incorrect tyre pressure can cause tyre failure, loss of vehicle control, and serious injury. Always consult your tyre manufacturer's specifications and a professional tyre installer for final pressure recommendations. Never exceed the maximum pressure rating marked on your tyre sidewall.
Original Tyre
Your current tyre
New Tyre
What you're changing to
How This Calculator Works
Load-Matching Principle
When you change tyre sizes, the new tyre may have a different load capacity. This calculator uses a load-matching approach: it determines what pressure your new tyre needs to carry the same weight as your original tyre at its current pressure.
Load Index
The load index is a number on your tyre sidewall (e.g., 91) that indicates the maximum load capacity at maximum pressure. A tyre with load index 91 can carry 615 kg at its maximum rated pressure. This calculator uses industry-standard load index tables from TRA (Tire and Rim Association).
Pressure-Load Relationship (TRA Formula)
A tyre's load capacity is not directly proportional to pressure. The TRA developed a formula where Load = MaxLoad × (Pressure/MaxPressure)^0.585. This exponent of 0.585 was developed by G. Hoover at TRA and accounts for the non-linear relationship. For example, at 32 PSI (of 36 PSI max), a tyre carries about 93% of its rated load. At 20 PSI, it carries only 71% of maximum capacity. This calculator uses this industry-standard formula.
The Calculation
The calculator performs these steps: (1) Looks up your original tyre's maximum load from its load index, (2) Calculates the actual load being carried at your current pressure using the TRA formula, (3) Looks up your new tyre's maximum load from its load index, (4) Uses the inverse TRA formula (P = Pmax × LoadRatio^(1/0.585)) to find the pressure needed for the new tyre to carry that same load.
Light Truck (LT) Tyres
LT tyres have different pressure-load characteristics than passenger tyres and are rated for higher pressures. They use Load Range designations (C, D, E) which indicate maximum pressure: Load Range C = 50 PSI, D = 65 PSI, E = 80 PSI. Each load range has its own pressure-load curve, which this calculator accounts for.