Calculating your overall diameter is straightforward once you know the Tire Size formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Tire Size Calculator.
What is Tire Size?
The Tire Size calculation tells you your overall diameter from a few simple inputs. The figure you are solving for here is the overall diameter.
The Tire Size formula
The core formula is:
Overall diameter = Rim diameter + 2 × (Tyre width × Aspect ratio ÷ 100) ÷ 25.4Here is what each input means:
- Tyre width — a value measured in mm. Example: 205 mm.
- Aspect ratio — a value measured in %. Example: 55 %.
- Rim diameter — a value measured in in. Example: 16 in.
How to calculate it step by step
- Write down the tyre width (for example, 205 mm).
- Write down the aspect ratio (for example, 55 %).
- Write down the rim diameter (for example, 16 in).
- Apply the formula above to get your overall diameter.
- Double-check the result with the Tire Size Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Tyre width | 205 mm |
| Aspect ratio | 55 % |
| Rim diameter | 16 in |
| Overall diameter | 24.88 |
| Sidewall height | 112.8 |
With tyre width of 205 mm, aspect ratio of 55 % and rim diameter of 16 in, the overall diameter works out to 24.88.
Example 2
With tyre width of 410 mm, aspect ratio of 55 % and rim diameter of 16 in, the overall diameter works out to 33.76.
| Result | Value |
|---|---|
| Overall diameter | 33.76 |
| Sidewall height | 225.5 |
Example 3
With tyre width of 100 mm, aspect ratio of 55 % and rim diameter of 16 in, the overall diameter works out to 20.33.
| Result | Value |
|---|---|
| Overall diameter | 20.33 |
| Sidewall height | 55.0 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Tire Size Calculator does it instantly, for free, with the formula and a worked example built in.
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