Tire Equivalent Size Calculator
Tire equivalent size calculator works out Tire size and Sidewall height from Original tire section width, Original tire aspect ratio and Original tire rim diameter. The default figures on this page give diameter difference of -0.08 %; 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
- Sidewall height h = 205 x 55 / 100 = 112.75 mm.
- Overall diameter d = 16 x 25.4 + 2 x 112.75 = 631.9 mm.
- Circumference c = pi x 631.9 = 1985.1724 mm.
- Revolutions per mile = 1,609,344 / 1985.1724 = 810.682.
- New tire diameter = 631.4 mm, a change of -0.5 mm (-0.0791%).
- At an indicated 60 mph the vehicle is really doing 59.953.
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 equivalent size 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 equivalent size calculator
Enter Original tire section width, Original tire aspect ratio, Original tire rim diameter and New tire section width. 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 Original tire section width, Original tire aspect ratio, Original tire rim diameter, New tire section width, New tire aspect ratio and New tire rim diameter.
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
What tire size can I substitute?
Converts a metric tire code into nominal sidewall height, overall diameter, circumference and revolutions per mile, decodes the load index and speed symbol. On this page it is computed from Original tire section width, Original tire aspect ratio and Original tire rim diameter and shown as Tire size, Sidewall height and Overall diameter.
Can you provide a chart that compares tire sizes?
Converts a metric tire code into nominal sidewall height, overall diameter, circumference and revolutions per mile, decodes the load index and speed symbol. Put your own figures in above and the working updates beside the result. For scale, the default figures on this page give -0.08 %.
What is the 3 tire rule?
On the figures this page opens with, the diameter difference is -0.08%. No single number fits everyone: it follows Original tire section width, Original tire aspect ratio and Original tire rim diameter, so enter your own above and the answer updates as you type.
How to convert 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 information do I need for the tire equivalent size calculator?
You need Original tire section width, Original tire aspect ratio, Original tire rim diameter, New tire section width, New tire aspect ratio, New tire rim diameter, Speed shown on the speedometer and Speed unit. The page opens with default figures that give diameter difference of -0.08 %, so you can see the scale before entering your own.
What does this tire equivalent size 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.