Tire Spec Calculator
Tire spec calculator works out Tire size and Sidewall height from Section width, Aspect ratio and Rim diameter. The default figures on this page give overall diameter of 676.40 mm; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
6 steps
- Decoded "P225/60R16 97H": section width 225 mm, aspect ratio 60%, rim 16 in.
- Sidewall height h = 225 x 60 / 100 = 135 mm.
- Overall diameter d = 16 x 25.4 + 2 x 135 = 676.4 mm.
- Circumference c = pi x 676.4 = 2124.9733 mm.
- Revolutions per mile = 1,609,344 / 2124.9733 = 757.348.
- Service description 97H: load index 97 = 730 kg per tire, speed symbol H = 210 km/h.
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the tire spec calculator calculates
Converts a metric tire code into nominal sidewall height, overall diameter, circumference and revolutions per mile, decodes the load index and speed symbol, and compares two sizes for diameter difference and speedometer error.
How to use this tire spec calculator
Enter Section width, Aspect ratio, Rim diameter and Full tire code. Tire size updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Tire size, Sidewall height and Overall diameter.
The method follows NHTSA, Tire terminology and size designation, NIST Guide to the SI, Appendix B.9 conversion factors, Titan Tire, international load index chart (ISO/ETRTO load index to kilograms), Goodyear, tire speed symbol table (A1 to T) and Tire Reference, speed rating chart (U, H, V, W, Y and the ZR marking).
What you can compare
Change one input at a time to see how it affects Tire size, Sidewall height and Overall diameter. The inputs you can vary are Section width, Aspect ratio, Rim diameter, Full tire code, New tire section width and New tire aspect ratio.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I find my tire specs?
Converts a metric tire code into nominal sidewall height, overall diameter, circumference and revolutions per mile, decodes the load index and speed symbol. Open “How this is worked out” beside the result to see each step for your own figures.
How much difference in tire size is acceptable in inches?
On the figures this page opens with — section width 205 mm, aspect ratio 55% and rim diameter 16 in — the overall diameter is 676.4 mm. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
How do I convert metric tire sizes to standard sizes?
It needs Section width, Aspect ratio, Rim diameter, Full tire code, New tire section width and New tire aspect ratio. Nothing else is required, and the figures stay in your browser.
What information do I need for the tire spec calculator?
You need Section width, Aspect ratio, Rim diameter, Full tire code, New tire section width, New tire aspect ratio, New tire rim diameter and Speed shown on the speedometer. The page opens with default figures that give overall diameter of 676.40 mm, so you can see the scale before entering your own.
What does this tire spec calculator show?
It shows Tire size, Sidewall height, Overall diameter, Section width, Circumference, Revolutions per mile, Revolutions per kilometre and Original tire diameter, with the working and any warnings for your entries.
Can I check how the tire spec calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.