Tire Diameter Calculator

A tire size like 225/45R17 gives the section width in millimetres (225), the aspect ratio as a percent (45), and the rim diameter in inches (17). Sidewall height equals width times aspect ratio divided by 100. Overall diameter equals the rim diameter converted to millimetres plus twice the sidewall height, because there is sidewall above and below the wheel.

The aspect ratio is the sidewall height expressed as a percentage of the section width. In 225/45R17 the 45 means the sidewall is 45 percent of 225 mm, or about 101 mm tall. A lower aspect ratio means a shorter, sportier sidewall; a higher number means a taller sidewall with more cushioning.

Revolutions per mile is how many times the tire turns to travel one mile, and it is set by the tire's circumference. Speedometers and odometers are calibrated to the original tire's revolutions per mile, so changing to a tire with a different diameter changes both readings. It also affects effective gearing and the distance covered per wheel rotation.

Tire and wheel guidance commonly suggests keeping a replacement tire's overall diameter within roughly 3 percent of the original to avoid significant speedometer error, clearance problems, and gearing changes. This calculator gives you the exact diameter so you can compare a new size against your factory size directly.

Yes. Enter the section width in millimetres, the aspect ratio as a number, and the rim diameter in inches for any P-metric or metric tire. To compare plus-size options, calculate each size and check that the overall diameters stay close, since plus-sizing aims to keep diameter constant while increasing wheel size and reducing sidewall.