How to use the Tire Size Calculator
Enter the section width, aspect ratio and rim diameter and the result appears straight away. Everything runs in your browser, so nothing you type is uploaded.
The formula
Diameter = rim × 25.4 + 2 × width × aspect ÷ 100 (mm)
Examples
- 225/45R17
Overall diameter: 24.97 in (634 mm) Sidewall height: 3.99 in (101 mm) Circumference: 78.5 in Revolutions per mile: 808
- 205/55R16
Overall diameter: 24.88 in (632 mm) Sidewall height: 4.44 in (113 mm) Circumference: 78.2 in Revolutions per mile: 811
- 265/70R17 truck tire
Overall diameter: 31.61 in (803 mm) Sidewall height: 7.30 in (186 mm) Circumference: 99.3 in Revolutions per mile: 638
Limitations and assumptions
Nominal size. Actual dimensions vary by brand, wear and pressure.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the section width in millimetres, 45 is the aspect ratio (sidewall height as a percent of width) and 17 is the rim diameter in inches.
How is overall diameter calculated?
Rim diameter × 25.4 plus two sidewalls, where each sidewall is width × aspect ratio ÷ 100, all in millimetres.
How much diameter change is safe?
A common guideline is to stay within about 3% of the original diameter so the speedometer and gearing stay close to stock.
Does it include tread wear?
No. It gives the new tire size.