Tire Size Conversion Calculator

Find sidewall height, overall diameter, and circumference from a standard tire size code.

⏱ Updated: 17 Sep 2026

Calculator

Free tire size calculator: find sidewall height, overall diameter, and circumference from a standard tire size, and compare two sizes for speedometer difference.

Sidewall
mm
Diameter
in
Circumference
in
Revs / Mile
Speedometer / Diameter Difference
%

Enter a tire size (width, aspect ratio, wheel diameter) to calculate its dimensions.

How to Use the Tire Size Conversion Calculator

Width, aspect ratio, and wheel diameter, the three numbers printed right on a tire's sidewall, convert directly into overall diameter and other real-world dimensions.

A tire size like 225/50R17 packs three numbers into one code: 225 is the tread width in millimeters, 50 is the sidewall's aspect ratio as a percentage of that width, and 17 is the wheel diameter in inches — three different units in three positions, which is exactly why converting it by hand is annoying.

sidewall height (mm) = width × (aspect ratio ÷ 100)
overall diameter (in) = wheel diameter + 2 × (sidewall ÷ 25.4)
circumference (in) = π × overall diameter
revolutions per mile = 63,360 ÷ circumference

This Tire Size Calculator runs all four, and optionally compares two sizes to estimate how far off your speedometer would read if you swapped — a bigger overall diameter means the wheel turns fewer times per mile than the speedometer assumes, so it under-reads your actual speed, and a smaller diameter over-reads it.