Tire size calculator
Type the size printed on your sidewall to see how tall and wide the tire really is — in inches and millimeters — plus sidewall height, circumference and revolutions per mile.
| Measurement | Imperial | Metric |
|---|---|---|
| Overall diameter (height) | 31.61 in | 803 mm |
| Section width | 10.43 in | 265 mm |
| Sidewall height | 7.30 in | 186 mm |
| Rim (wheel) diameter | 17 in | 432 mm |
| Aspect ratio | 70% | — |
| Circumference | 99.3 in | 2522 mm |
| Revolutions | 638 per mile | 396 per km |
How to read the result
Overall diameter is the height of the tire from the ground to the top of the tread. It drives speedometer accuracy, ground clearance and effective gearing, so it is the number to watch when changing sizes.
Section width is the nominal width of the inflated tire at its widest point, measured on the design rim width. It is not the tread width, and it varies with the width of the wheel it is mounted on (a common rule of thumb is about 5 mm per half inch of rim width).
Sidewall height is the distance from the wheel lip to the tread. A taller sidewall rides softer and protects the wheel; a shorter one steers more sharply.
Revolutions per mile come from the circumference. Fewer revolutions per mile means the speedometer reads low and the engine turns slower at the same road speed.
Worked example: 245/75R16
- Width 245 mm × 75% aspect ratio = 183.75 mm sidewall.
- 183.75 mm ÷ 25.4 = 7.234 in.
- Diameter = 16 in wheel + 2 × 7.234 in = 30.47 in (774 mm).
- Circumference = π × 30.47 = 95.7 in, so about 662 revolutions per mile.
All formulas are listed on the methodology page.
Frequently asked questions
How do I calculate tire diameter from the size?
Multiply the width (mm) by the aspect ratio to get the sidewall, convert to inches (÷ 25.4), double it and add the rim diameter. For 245/75R16: 245 × 0.75 = 183.75 mm = 7.234 in; 2 × 7.234 + 16 = 30.47 in.
Why does the measured height of my tire differ from the calculator?
The calculator gives nominal (design) dimensions of a new, unloaded tire. Actual height varies by manufacturer and model, shrinks as tread wears, and the loaded radius is smaller than half the diameter because the tire flattens where it meets the road.
What does revolutions per mile mean?
How many times the tire turns to cover one mile, from its circumference. 245/75R16 has a circumference of 95.7 in, so it turns about 662 times per mile. Tire makers publish a measured figure that is slightly higher because of deflection under load.
Can I enter LT and P-metric sizes?
Yes. The P (passenger) and LT (light truck) prefixes do not change the geometry, so 265/70R17, P265/70R17 and LT265/70R17 have the same nominal dimensions. They differ in construction and load capacity — check the load index.
Does it work with inch sizes like 35x12.50R20?
Yes. Flotation sizes give diameter × width directly; 35x12.50R20 is 35 in tall, 12.50 in wide and has a 7.50 in sidewall on a 20-inch wheel.
Related tools
- Tire size comparison
Two sizes side by side, to scale, with speedometer and RPM tables.
- Inch ↔ metric converter
33x12.50R17 to metric and back, with closest equivalents.
- How to read a tire size
Every part of a sidewall code explained.
- Tire size chart
Common sizes by wheel diameter with heights and widths.