Tire Diameter Calculator
Tire diameter in millimetres = rim × 25.4 + 2 × width × aspect ratio ÷ 100. For 225/45R17: 17 × 25.4 + 2 × 225 × 0.45 = 634.3 mm, or 25.0 inches. Enter any size below for its height, sidewall, circumference and revolutions per mile and km.
Tire diameter from size
The calculator needs JavaScript. The formulas and tables on this page work without it: sidewall = width × aspect ratio ÷ 100, and diameter = rim × 25.4 + 2 × sidewall (in mm).
Overall diameter
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Formulas
| Value | Formula | Example: 265/70R17 |
|---|---|---|
| Sidewall height | width × aspect ratio ÷ 100 | 265 × 70 ÷ 100 = 185.5 mm (7.30 in) |
| Overall diameter | rim × 25.4 + 2 × sidewall | 17 × 25.4 + 2 × 185.5 = 802.8 mm (31.6 in) |
| Circumference | π × diameter | 3.1416 × 802.8 = 2522 mm (99.3 in) |
| Revolutions per mile | 63,360 ÷ circumference in inches | 63,360 ÷ 99.3 = 638 |
| Actual speed | speedometer × new diameter ÷ old diameter | 60 × 830.8 ÷ 802.8 = 62.1 mph on 285/70R17 |
These are nominal dimensions from the size code. Real tires differ slightly by make, model, tread depth and the rim they are fitted to.
Diameter of common sizes
| Size | Diameter (in) | Diameter (mm) | Width (in) | Sidewall | Revs per mile |
|---|---|---|---|---|---|
| 195/65R15 | 25.0 in | 634 mm | 7.7 in | 127 mm | 807 |
| 205/55R16 | 24.9 in | 632 mm | 8.1 in | 113 mm | 811 |
| 225/45R17 | 25.0 in | 634 mm | 8.9 in | 101 mm | 808 |
| 225/40R18 | 25.1 in | 637 mm | 8.9 in | 90 mm | 804 |
| 235/55R19 | 29.2 in | 741 mm | 9.3 in | 129 mm | 691 |
| 245/45R20 | 28.7 in | 728 mm | 9.6 in | 110 mm | 703 |
| 265/70R17 | 31.6 in | 803 mm | 10.4 in | 186 mm | 638 |
| 275/55R20 | 31.9 in | 810 mm | 10.8 in | 151 mm | 632 |
| 285/75R16 | 32.8 in | 834 mm | 11.2 in | 214 mm | 614 |
Nominal and real diameter
The size code gives a nominal diameter. Tire makers publish the real overall diameter of each tire model in their data sheets; it can differ by a few millimetres, and a worn tire is smaller than a new one because the tread is lower.
Frequently asked questions
How do I calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall in millimetres. Double it and add the rim diameter times 25.4. Divide by 25.4 for inches.
What is the diameter of a 225/45R17 tire?
About 634 mm, or 25.0 inches, from the nominal size. The sidewall is 101.25 mm (4.0 in) and the rim is 17 inches. Real tires of this size differ by a few millimetres between models.
How tall is a 265/70R17 tire?
About 803 mm, or 31.6 inches, from the nominal size. The sidewall is 185.5 mm (7.3 in). Real tires of this size differ slightly between makes and models, and a worn tire is a little smaller.
What is the circumference of a tire?
Circumference is π times the diameter. For 265/70R17 it is about 2522 mm (99.3 in), which means about 638 revolutions per mile or 397 per kilometre at the nominal size.
Does tread wear change the diameter?
Yes. As the tread wears, the diameter shrinks by twice the lost tread depth. A tire that has lost 6 mm of tread is about 12 mm smaller in diameter.
Why does the tire maker list a different diameter?
Makers measure their real tires on a specific rim and publish those figures. The size code gives a nominal value, which is a good guide but not exact.
Last updated: October 15, 2026