Tire Size Formula Explainer
See each step of the math for a size. Use the calculator below. Results update as you type.
How the Tire Size Formula Explainer Works
This explainer works by showing every calculation step from the size code to the final dimensions, with the working shown. It is a step-by-step math walkthrough.
- Enter a size, for example 225/45R17.
- Read each step: sidewall in mm, sidewall in inches, rim in mm, diameter, circumference, radius and revolutions.
- Read the working for each step.
- Change the size to see how each step changes.
The Formula
The formula is the full set of 9 tire equations in order.
1. Sidewall (mm) = W × AR ÷ 100 2. Sidewall (in) = mm ÷ 25.4 3. Rim (mm) = Rim × 25.4 4. D (mm) = Rim (mm) + 2 × Sidewall (mm) 5. D (in) = D (mm) ÷ 25.4 6. C = π × D 7. Radius = D ÷ 2 8. Revs per Mile = 63,360 ÷ C (in) 9. Revs per km = 1,000,000 ÷ C (mm)Worked example: 225/45R17 gives 101.25 mm, 3.99 in, 431.8 mm, 634.3 mm, 24.97 in, 78.45 in, 12.49 in, 807.6 and 501.8.
Conversion Table
The conversion table lists all 9 steps for 225/45R17 with the formula, the working and the result. Each step uses the result of an earlier step.
Steps 1 and 4 hold the core logic. Every other step is a unit change or a property of a circle.
| Step | Formula | Working | Result |
|---|---|---|---|
| 1. Sidewall (mm) | W × AR ÷ 100 | 225 × 45 ÷ 100 | 101.25 mm |
| 2. Sidewall (in) | mm ÷ 25.4 | 101.25 ÷ 25.4 | 3.99 in |
| 3. Rim (mm) | Rim × 25.4 | 17 × 25.4 | 431.8 mm |
| 4. Diameter (mm) | Rim + 2 × Sidewall | 431.8 + 202.5 | 634.3 mm |
| 5. Diameter (in) | mm ÷ 25.4 | 634.3 ÷ 25.4 | 24.97 in |
| 6. Circumference | π × D | 3.1416 × 24.97 | 78.45 in |
| 7. Radius | D ÷ 2 | 24.97 ÷ 2 | 12.49 in |
| 8. Revs per mile | 63,360 ÷ C | 63,360 ÷ 78.45 | 807.6 |
| 9. Revs per km | 1,000,000 ÷ C(mm) | 1,000,000 ÷ 1993 | 501.8 |
What Is the Tire Size Formula Explainer?
The Tire Size Formula Explainer is a tool that shows the inputs and calculation steps behind every tire size result. It suits students, new tire techs and anyone who wants to check the math.
Showing the steps builds trust in the results and makes errors in hand calculations easy to find.
Common Uses
This explainer has 4 uses: training, homework, result checking and content writing.
- Train staff: Teach tire math to new hires.
- Solve homework: Show working for school problems.
- Check results: Verify another calculator's output.
- Write content: Quote correct formulas in articles.