How to calculate tire diameter
For metric sizes such as 265/70R17, the sidewall height is the width times the aspect ratio. Add two sidewalls to the wheel diameter to get the overall height. Flotation sizes such as 33x12.50R20 already state the diameter: 33 inches tall, 12.50 inches wide, for a 20 inch wheel.
Tire diameter formula
Diameter (in) = rim (in) + 2 × sidewall
Circumference = π × diameter
Revolutions per mile = 63,360 ÷ circumference (in)
Worked examples
- 265/70R17: sidewall 265 × 0.70 = 185.5 mm, or 7.30 in. Diameter 17 + 2 × 7.30 = 31.61 in (803 mm). Circumference 99.29 in, about 638 revolutions per mile.
- 33x12.50R20: 33 in tall, so each sidewall is (33 − 20) ÷ 2 = 6.5 in. Circumference 103.67 in, about 611 revolutions per mile.
Diameter of popular tire sizes
| Size | Diameter (in) | Diameter (mm) | Revs per mile |
|---|---|---|---|
| 205/55R16 | 24.9 | 632 | 811 |
| 215/55R17 | 26.3 | 668 | 767 |
| 225/45R17 | 25.0 | 634 | 808 |
| 225/65R17 | 28.5 | 724 | 707 |
| 235/65R17 | 29.0 | 737 | 695 |
| 245/40R18 | 25.7 | 653 | 784 |
| 245/75R16 | 30.5 | 774 | 662 |
| 265/70R17 | 31.6 | 803 | 638 |
| 275/55R20 | 31.9 | 810 | 632 |
| 275/70R18 | 33.2 | 842 | 608 |
| 285/70R17 | 32.7 | 831 | 617 |
| 285/75R17 | 33.8 | 859 | 596 |
| 295/70R18 | 34.3 | 870 | 589 |
| 315/70R17 | 34.4 | 873 | 587 |
Comparing two sizes? The tire size calculator shows the difference and your speedometer error. Bigger tires? Check your real speed with the speedometer calculator and gearing with the gear ratio calculator.
Frequently asked questions
How do I find my tire diameter?
For a metric size, multiply the width in mm by the aspect ratio, divide by 100 and by 25.4 to get the sidewall in inches, double it and add the rim size. A 265/70R17 tire is about 31.6 inches tall.
What size is a 33 inch tire in metric?
Common metric sizes close to 33 inches are 285/70R17 (32.7 in), 275/70R18 (33.2 in) and 285/75R17 (33.8 in). 285/75R16 is about 32.8 inches.
How many revolutions per mile does a tire make?
Divide 63,360 (inches in a mile) by the tire's circumference in inches. A 31.6 inch tire turns about 638 times per mile.
Is the actual tire diameter the same as the calculated one?
Close, but not exact. Brands differ slightly, and a worn tire is shorter than a new one by roughly twice the lost tread depth, often 0.3 to 0.5 inches over its life.
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Last updated October 2026.