Tire Size Calculator
Enter width, aspect ratio, and rim to get overall diameter, sidewall, circumference, and revs per mile.
TireSize works entirely in your browser — no size is uploaded or stored.Try: Section width (mm)=205, Aspect ratio (%)=55, Rim diameter (in)=16 → 24.88, 4.44, 78.16, 811
How to use
A tire size like 205/55R16 reads as: 205 = section width in millimetres, 55 = aspect ratio (sidewall height is 55% of the width), and 16 = rim diameter in inches. The calculator turns those three numbers into overall diameter, sidewall height, circumference, and how many times the wheel turns per mile.
Overall diameter = rim x 25.4 + 2 x (width x aspect / 100) in millimetres. Circumference = diameter x pi, and revolutions per mile = 63,360 / circumference-inches.
FAQ
How do I read a tire size like 205/55R16?
205 is the tread width in mm, 55 is the sidewall height as a percentage of that width, and 16 is the wheel diameter in inches. The R means radial construction.
Why does overall diameter matter?
Diameter drives ground clearance, gearing, and speedometer accuracy. Changing it by more than about 3% can throw off your speedo and rub the fenders.
What is revolutions per mile for?
Revs per mile tells you how many wheel turns cover one mile. It is what your speedometer and odometer are calibrated against, so it predicts speedometer error when you change sizes.