Tire Diameter Calculator

How do you calculate the diameter of a tire?

Tire dimensions are usually given on the sidewall of the tire in a format like this: 235/75R15. This indicates the width, aspect ratio, and wheel diameter.

• The first number, 235, is the width of the tire in millimeters.
• The second number, 75, is the aspect ratio – which is the height of the tire sidewall as a percentage of the width. So, the sidewall height of this tire is 75% of 235mm.
• The last number, 15, is the diameter of the wheel in inches that the tire is designed to fit.

How do you Convert the diameter of a tire?

1. Convert the width to inches (since the wheel diameter is in inches). 1 inch = 25.4 mm. So, 235mm ÷ 25.4 = 9.25 inches.
2. Calculate the sidewall height: this is the width times the aspect ratio, then divided by 100 (because the aspect ratio is a percentage). So, 9.25 inches * 75 / 100 = 6.9375 inches.
3. The overall diameter is twice the sidewall height (one for the top and one for the bottom) plus the diameter of the wheel. So, 2 * 6.9375 inches + 15 inches = 29.875 inches.