Tire sizes printed on the sidewall use a mix of millimetres and inches: a 225/45R17 tire is 225 mm wide, has a sidewall 45% as tall as it is wide, and fits a 17-inch wheel. What the code does not tell you directly is how tall the tire is — the overall diameter that decides ride height, gearing and speedometer accuracy. This calculator converts the three numbers into overall diameter, sidewall height, circumference and revolutions per mile.
How to use the tire diameter calculator
- Find the size on the sidewall, written like 225/45R17.
- Enter the section width (225), the aspect ratio (45) and the rim diameter (17 in).
- Read the results. Switch the units beside each result for millimetres, centimetres or feet.
To compare two sizes directly, use the tire size comparison calculator, and to see what a diameter change does to your speedometer try the speedometer error calculator.
Tire diameter formula
Overall diameter = rim diameter + 2 × sidewall height
Circumference = π × overall diameter
Revolutions per mile = 63,360 in ÷ circumference
- Width is in millimetres, so convert the sidewall to inches by dividing by 25.4.
- Aspect ratio is a percentage of the width, not an absolute height.
- 63,360 is the number of inches in a mile (5,280 ft × 12).
Worked example: 225/45R17
- Sidewall height: 225 × 45 ÷ 100 = 101.25 mm = 3.99 in.
- Overall diameter: 17 + 2 × 3.986 = 24.97 in (634 mm).
- Circumference: π × 24.97 = 78.4 in.
- Revolutions per mile: 63,360 ÷ 78.4 = 808.
This agrees with the 24.97 in published for the same size by tire retailers.
Diameters of common sizes
Each row is calculated with the formula above.
| Tire size | Sidewall height | Overall diameter | Diameter (mm) | Revolutions per mile |
|---|---|---|---|---|
| 205/55R16 | 112.8 mm | 24.88 in | 632 mm | 811 |
| 215/60R16 | 129.0 mm | 26.16 in | 664 mm | 771 |
| 225/45R17 | 101.2 mm | 24.97 in | 634 mm | 808 |
| 235/55R18 | 129.2 mm | 28.18 in | 716 mm | 716 |
| 245/40R19 | 98.0 mm | 26.72 in | 679 mm | 755 |
| 265/70R17 | 185.5 mm | 31.61 in | 803 mm | 638 |
| 275/55R20 | 151.2 mm | 31.91 in | 810 mm | 632 |
Why diameter matters
A larger overall diameter raises the vehicle slightly and, because each revolution covers more ground, makes the engine turn slower at a given road speed and makes the speedometer read low. A smaller diameter does the opposite. Plus-sizing — fitting a bigger rim with a lower-profile tire — is usually done by keeping the overall diameter within about 3% of the original so that the speedometer, ABS and traction systems keep working as designed. Off-road drivers go the other way and choose taller tires for ground clearance, then re-gear the axles to compensate; the gear ratio calculator helps with that part.
Remember that the tire’s actual diameter changes a little with wear (a new tread is about 8–10 mm deep on a passenger tire, so a worn tire is about 0.6 in smaller overall) and with load and pressure. The calculation here is the nominal figure used by tire manufacturers and retailers; the manufacturer’s published specification for your exact model is the best source when fit is critical.
Sources
The size markings follow the ISO metric system used by the Tire and Rim Association and the US Department of Transportation tire label. The check value for 225/45R17 (24.97 in) comes from the tire-size reference data collected from tiresize.com on 2026-10-09.
Frequently asked questions
How do I calculate tire diameter from 225/45R17?
Multiply the width by the aspect ratio (225 × 0.45 = 101.25 mm), convert it to inches (3.99 in), double it for the two sidewalls (7.97 in) and add the rim diameter: 7.97 + 17 = 24.97 inches.
What does the number after R mean?
It is the wheel (rim) diameter in inches. The R stands for radial construction.
Does the calculated diameter match the real tire?
It is the nominal diameter. A mounted tire on a measuring rim can differ by about ±1–2% depending on the brand, inflation and tread wear, so use it for comparisons rather than clearance checks.
Why do revolutions per mile matter?
Revolutions per mile (63,360 inches ÷ circumference) tell you how many times the tire turns over a mile. It is the number used to set speedometers and compute gearing.
How do I convert the diameter to millimetres?
Multiply inches by 25.4, or use the unit menu beside the result.