Tire Size Calculator

Enter every measurement Tire Size Calculator asks for, then press Calculate. The result stays blank while you are still typing.

Press Calculate after the entries are in.

What Tire Size is for

Tire Size Calculator is the page to open when you need tire size from measurements you already have. It belongs with the other automotive calculators because that is the work this arithmetic supports. Type the entries first. The result stays hidden until you press Calculate, so a half-finished form never flashes a number you might mistake for the answer.

A 225/45R17 tire is 225 mm wide, with a sidewall 45 percent of that width, on a 17 inch rim.

How to fill in this calculator

Section width (mm). Enter the value this part of the job needs.
Aspect ratio. Enter the value this part of the job needs.
Wheel diameter (in). Enter the value this part of the job needs.

The formula in plain language

Overall diameter is the wheel diameter plus two sidewalls. Sidewall height is the section width times the aspect ratio. A new tire that is taller than the old one makes the speedometer read low and adds a little ride height. Offset is how far the mounting face sits from the wheel's centerline. A lower offset pokes the wheel outward.

A worked example

A 225/60R16 tire has a sidewall of 135 mm and an overall diameter of about 26.6 inches. Moving to a 235/65R17 grows the diameter by roughly 5%, so a speedometer that shows 60 mph is actually closer to 63.

Road speed, engine rpm, and tire diameter are one drivetrain. After this tire size result, two tire sizes compared and tire size conversion should be read with the same tire and the same gear.

Mistakes that move the answer

Compare overall diameter, not just the wheel inch size.
A wider wheel with the same offset sticks out farther by half the extra width.

How to check the result

After Calculate shows the tire size result, redo the last operation with a pencil using the rounded figures on the screen. Matching that check means the inputs were read correctly. If you edit a label or a sentence in this HTML file, reload the page. The form and this article both live in the file, so the edit is what visitors will read.