Size Tire Calculator
Size of tire 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
4 steps
- 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.
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the size of tire 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 size of tire calculator
Enter Section width, Aspect ratio, Rim diameter and Full tire code. Tire size updates as you type, and “Use a worked example” loads checked figures you can adjust. 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).
205/55R16 sidewall height and diameter example
With Section width: 205, Aspect ratio: 55, Rim diameter: 16, the calculator returns Sidewall height: 112.75, Overall diameter: 631.9. This example is checked against an independent source in the test suite.
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, Original tire section width and Original 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 you calculate your tire size?
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.
What is the 3 tire rule?
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 much difference in tire size is acceptable in inches?
It needs Section width, Aspect ratio, Rim diameter, Full tire code, Original tire section width and Original tire aspect ratio. Nothing else is required, and the figures stay in your browser.
Can you provide a tire size calculator and comparison tool?
Converts a metric tire code into nominal sidewall height, overall diameter, circumference and revolutions per mile, decodes the load index and speed symbol. 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. Put your own figures in their place and the working updates beside the result.
What is the 3% rule for tire sizes?
The result gives Tire size, Sidewall height, Overall diameter, Section width, Circumference and Revolutions per mile, each labelled with its unit so the figure can be read on its own.
What information do I need for the size of tire calculator?
You need Section width, Aspect ratio, Rim diameter, Full tire code, Original tire section width, Original tire aspect ratio, Original tire rim diameter and New tire section width. The page opens with default figures that give overall diameter of 676.40 mm, so you can see the scale before entering your own.