How the tire size calculator works
A metric tire size such as 225/45R17 contains everything needed to work out the tire's dimensions:
- Sidewall height = width × aspect ratio ÷ 100 = 225 × 45 ÷ 100 = 101.25 mm (3.99 in)
- Overall diameter = rim diameter + 2 × sidewall = 17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 in)
- Circumference = π × diameter = 3.14159 × 634.3 = 1,992.7 mm (78.45 in)
- Revolutions per mile = 1,609,344 mm ÷ 1,992.7 mm = 807.6 (501.8 per km)
One inch is exactly 25.4 mm and one mile is exactly 1,609.344 m, so the only rounding happens in the displayed results.
Worked example: 265/70R17
A common pickup and SUV size. Sidewall = 265 × 0.70 = 185.5 mm (7.30 in). Diameter = 431.8 + 2 × 185.5 = 802.8 mm, which is 31.6 inches — that is why 265/70R17 is often called a "32-inch tire". It turns about 638 times per mile.
Diameter of common tire sizes
| Size | Diameter (in) | Diameter (mm) | Sidewall (in) | Revs/mile |
|---|---|---|---|---|
| 195/65R15 | 25.0 | 635 | 4.99 | 807 |
| 205/55R16 | 24.9 | 632 | 4.44 | 811 |
| 215/55R17 | 26.3 | 668 | 4.66 | 767 |
| 225/45R17 | 25.0 | 634 | 3.99 | 808 |
| 225/40R18 | 25.1 | 637 | 3.54 | 804 |
| 235/45R18 | 26.3 | 669 | 4.16 | 766 |
| 255/35R19 | 26.0 | 661 | 3.51 | 775 |
| 265/70R17 | 31.6 | 803 | 7.30 | 638 |
| 275/55R20 | 31.9 | 811 | 5.95 | 632 |
Browse all tire sizes for a page per size with equivalents and the vehicles that use it.
What the size does not tell you
Two tires with the same size can differ slightly in real diameter and tread width. The size also says nothing about load capacity or speed capability — check the load index and speed rating on the sidewall and the minimum values on your vehicle's tire placard (usually on the driver's door jamb).
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the section width in millimetres, 45 is the aspect ratio (the sidewall is 45 % as tall as the tire is wide), R means radial construction and 17 is the wheel (rim) diameter in inches. Any load index and speed rating that follows, such as 91W, does not change the dimensions.
Is the calculated diameter exactly what I will measure?
Close, but not exact. The calculation gives the nominal design size. Real tires vary by a few millimetres between brands and models, are slightly smaller under load, and change a little with inflation pressure and wear.
What does revolutions per mile mean?
It is how many times the tire turns to cover one mile. Speedometers and odometers count wheel revolutions, so a tire with fewer revolutions per mile (a taller tire) makes them read low.
Does the P or LT in front of the size matter?
Not for dimensions. P (passenger) and LT (light truck) tires of the same size have the same nominal diameter and width, but LT tires have different load ratings and inflation pressures. Only fit what your vehicle placard allows.
Can I enter flotation sizes such as 33x12.50R15?
Yes. Flotation sizes give the diameter and width directly in inches, so the calculator uses them as written and works out the sidewall height and equivalent aspect ratio.