Tyre size guide
The marking on a tyre sidewall describes its nominal width, sidewall profile, construction and rim diameter. It does not identify a vehicle or guarantee that the tyre is approved for a particular wheel.
Size breakdown
- Nominal width
- —
- Aspect ratio
- —
- Rim diameter
- —
- Calculated diameter
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Check the complete sidewall for construction, load index and speed rating, then confirm vehicle and rim approval.
Worked example: 225/45R18
| Mark | Meaning | Worked value |
|---|---|---|
| 225 | Nominal section width | 225 mm |
| 45 | Sidewall height as a percentage of width | 101.25 mm |
| R | Radial construction | Radial tyre |
| 18 | Rim bead-seat diameter | 18 inches |
Diameter formula
Sidewall height is width multiplied by the aspect ratio and divided by 100. Overall diameter is rim inches multiplied by 25.4, plus two sidewalls. For 225/45R18, the calculation is 457.2 + 202.5 = 659.7 mm.
The aspect ratio is not a millimetre measurement. A 45-profile sidewall is taller on a 245 mm tyre than on a 205 mm tyre because it is 45 percent of a different width. That is why changing width without changing profile can also change rolling diameter and speedometer behaviour.
What the marking leaves out
The basic size does not prove wheel-width compatibility, load capacity, speed rating, arch clearance, strut clearance or manufacturer approval. Two tyres carrying the same nominal size can also vary slightly in measured width and diameter by model and approved rim width.
Use the tyre size comparison to compare geometry, the tyre width chart to understand rim-width ranges, and the wheel fitment guide for the order of checks. None replaces a physical clearance check or verified vehicle documentation.
Common questions
What does 225/45R18 mean?
It declares 225 mm nominal width, a sidewall height equal to 45 percent of that width, radial construction and an 18-inch rim diameter.
Is tyre profile measured in millimetres?
No. Profile is a percentage. The decoder converts that percentage into a calculated millimetre sidewall height.
Does the same tyre size fit every car?
No. The marking alone cannot establish wheel width, load, clearance or vehicle approval. Use verified vehicle documentation and measure the installed geometry.
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.