Tire size calculator
Turn a tire size like 265/70R17 into its overall diameter, width, sidewall height, circumference and revolutions per mile. It reads metric sizes (P, LT and plain) and flotation sizes.
Example: 265/70R17 dimensions
| Measurement | Inches | Millimetres |
|---|---|---|
| Overall diameter | 31.61 | 803 |
| Section width | 10.43 | 265 |
| Sidewall height | 7.30 | 185.5 |
| Rim diameter | 17.0 | 432 |
| Radius (axle to ground, unloaded) | 15.80 | 401 |
| Circumference | 99.29 | 2,522 |
Revolutions per mile: 638.1
Nominal design dimensions from the size code. A mounted, inflated and loaded tire measures a little differently, and the same size varies by brand and tread.
Check the vehicle's door placard and owner's manual, and a tire professional, before changing sizes.
How to read a tire size
In 265/70R17, 265 is the section width in millimetres, 70 is the aspect ratio (the sidewall is 70% as tall as the tire is wide), R means radial construction, and 17 is the rim diameter in inches. A leading P (passenger) or LT (light truck) changes the load rating, not the dimensions.
Flotation sizes such as 33x12.50R15 state the overall diameter (33 in) and width (12.50 in) directly, with the rim size last.
The formulas
- Sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4
- Overall diameter (in) = 2 × sidewall + rim diameter
- Circumference (in) = π × diameter
- Revolutions per mile = 63,360 ÷ circumference (a mile is 63,360 inches)
Frequently asked questions
How tall is a 265/70R17 tire?
About 31.6 inches: the sidewall is 265 x 0.70 = 185.5 mm (7.30 in), twice that is 14.6 in, plus the 17 in rim.
What does the aspect ratio mean?
It is the sidewall height as a percentage of the section width. A 70 series tire has a sidewall 70% as tall as the tire is wide, so the same aspect on a wider tire means a taller sidewall.
Why does revolutions per mile matter?
The speedometer and odometer count wheel turns. A size with fewer revolutions per mile covers more ground per turn, so the gauges read low after the change. The size comparison calculator shows by how much.
Is the calculated diameter the real measured height?
It is the nominal design size. The loaded radius is smaller than half the diameter, and actual tires of the same size differ by a few tenths of an inch between brands and tread patterns.
Other calculators on this site
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Tire size comparison
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Effective gear ratio and RPM
How a tire size change shifts the effective axle ratio and engine RPM at a given speed.