Tire size math, without the guesswork
Enter two tire sizes and see the difference in diameter, width, sidewall and speedometer reading, with every formula shown.
Reading 225/65R17
- 225 section width in millimeters
- 65 sidewall height as % of width
- R radial construction
- 17 wheel diameter in inches
Diameter = 2 × 225 × 0.65 ÷ 25.4 + 17 = 28.5 in. Try the calculator.
Calculators
CalculatorLoad index and speed ratingWhat 91V or 116T means in pounds and mph.CalculatorSpeedometer error calculatorIndicated vs actual speed after a tire size change.CalculatorTire size calculatorDiameter, width, sidewall and revs per mile from a size code.CalculatorTire size comparisonDiameter, width, speedometer and 3% check for two sizes.
What we do and don't do
Math, shown
Every result shows the formula, so you can check it against the tire maker's spec sheet.
No fitment guesses
We never claim a size fits a vehicle. Your door placard and a tire shop decide that.
Official sources
Sidewall codes, tire age and maintenance guidance come from NHTSA and tire maker bulletins.
Guides
GuideCommon tire sizes chartDiameter, width and sidewall for 30 common sizes.GuideHow to read a tire sidewallSize, load, speed, DOT date code and UTQG grades decoded.GuidePlus sizing rulesThe four checks before changing tire or wheel size.GuideTire age and DOT date codeRead the week and year a tire was made, and why age matters.GuideTread depth and pressureMonthly pressure and tread checks, per NHTSA.
New to the terms? Our glossary explains them in plain words.