Tyre Diameter Calculator

When to use it

• Compare how a change in section width affects overall diameter before committing to parts, setup, or strategy changes.
• Check a workshop, dyno, simulator, or trackside spreadsheet result using a transparent formula: Rim diameter + two sidewalls.
• Use the result before changing wheels, tyres, brake hardware, or pressure targets. Track temperature, tyre construction, pad compound, surface grip, aero load, and driver technique can all change the real outcome.

How to use it

1. Enter section width in mm; the default value shown is 245 mm.
2. Enter aspect ratio in %; the default value shown is 40 %.
3. Enter wheel diameter in in; the default value shown is 18 in.

How to read the result

Overall diameter is shown in mm. Use the number as a calculated reference point, then compare it with measured data from the car, the part supplier, or the race environment. A mathematically clean answer can still be unsuitable if the inputs are estimated, rounded, measured under different conditions, or outside the normal range for the vehicle.

Assumptions and checks

• The calculator applies the displayed formula directly and assumes the entered section width, aspect ratio, and wheel diameter are valid for the vehicle and conditions being considered.
• Changing section width and aspect ratio together can move the answer in ways that are easy to miss, so make related setup changes deliberately.
• Measure clearances on the actual vehicle because wheel and tyre shapes vary between manufacturers.
• Check brake calculations against pad data, disc radius, hydraulic layout, and balance-bar setup.
• Recheck tyre pressures hot and cold at the circuit rather than relying only on a static estimate.