Tire & wheel tools
Tire size calculator
Enter a metric size like 205/55R16 — or fill width, aspect ratio and rim diameter separately — for the theoretical overall diameter, sidewall height, circumference and revolutions per km and per mile, shown in millimetres or inches.
Overall diameter
25.0 in
225/45R17
225/45R17 has a theoretical diameter of 25.0 in and a circumference of 78.5 in.
| Section width | 8.9 in |
|---|---|
| Sidewall height | 4.0 in |
| Overall diameter | 25.0 in |
| Circumference | 78.5 in |
| Revs per km | 501.8 |
| Revs per mile | 807.6 |
How it is calculated
Sidewall = width × aspect ratio ÷ 100; overall diameter = rim inches × 25.4 + 2 × sidewall; circumference = π × diameter; revs per km = 1,000,000 ÷ circumference (mm) and revs per mile = 1,609,344 ÷ circumference.
Math uses unrounded intermediate values and rounds only for display. Results are the size's theoretical diameter — real tires vary with brand, load, pressure and wear.
Diagrams are drawn to scale from the entered size and show geometry only, not a real profile or product.
How to fill each field
- Size codeA valid code overrides the three fields below; leave it blank to use the fields. Metric notations like 205/55R16, P225/45R17 and 205/55-16 work; flotation sizes like 31×10.50R15 do not.
- Aspect ratioA higher number means a taller sidewall. Lower profiles usually pair with larger rims and tighter diameter tolerances.
- Display unitMillimetres are native to the size code; inches help when cross-checking aftermarket data. Switching changes only the readout.
FAQ
What is the theoretical diameter of a 205/55R16?
Sidewall 205×0.55 = 112.75mm, diameter 16×25.4 + 2×112.75 = 631.9mm (24.9in). Run the size here for circumference and revs per mile too.
What are circumference and revs per mile for?
A diameter change alters how far the car travels per wheel revolution — that shifts speedometer and odometer readings and the effective gearing. Revs per mile feeds transmission calibration.
Is this the tire's real size?
No — it is the theoretical diameter of the size code. Measured diameters differ by a few millimetres between brands, and the rolling radius shrinks under load. Fine for comparisons; check manufacturer data for calibration.
What does aspect ratio 55 mean?
The sidewall height equals 55% of the section width — about 112.75mm on a 205. A 45 profile on the same width runs shorter.
Are flotation sizes supported?
Not yet — sizes like 31×10.50R15 are outside the parser. Convert the width to millimetres (inches × 25.4) and use the separate fields for an approximation.