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Tires & wheels

Tire Size Calculator

Calculate nominal tire diameter, sidewall height, circumference, and revolutions per mile.

By AutomotiveCalc · Published September 9, 2026
How this calculator is checked

Automated checks cover formula examples and input validation. This page has not received an independent automotive professional review.

Sources & calculation method · Review standards · Report a problem

Your numbers

Example calculation

Example results are shown below. Controls become available when the calculator loads.

Have a size code? Apply it here, or edit the dimensions below. Supports R or ZR metric sizes, including 225/45R17 91W and 225/45ZR17 (94Y) XL. Only dimensions are applied; load, speed and XL/RF markings are not evaluated. Size codes do not confirm vehicle fitment. How to read tire sizes →

Nominal diameter24.97 in
Sidewall height101.25 mm
Circumference78.45 in
Geometric revolutions per mile807.61 rev/mile

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How to use this calculator

Replace the example values with your own measurements or specifications. Keep the units shown beside each label. Results update as you type; reset restores the example.

These are nominal unloaded dimensions. Actual dimensions and rolling circumference depend on the tire model, load, pressure, and wear. This tool does not confirm vehicle fitment, load rating, speed rating, clearance, or wheel compatibility.

The formula

Sidewall (mm) = width × aspect ratio ÷ 100. Diameter (in) = wheel diameter + 2 × sidewall ÷ 25.4. Circumference = π × diameter.

A worked example

For a 225/45R17 tire, sidewall height is 101.25 mm and nominal diameter is 24.97 inches. The R denotes radial construction and does not enter the geometry formula.

Common questions

Does a matching diameter prove a tire will fit?

No. Check approved sizes, wheel width, load and speed ratings, and clearance with the vehicle and tire manufacturers.

Why do manufacturer revolutions per mile differ?

Published rolling measurements reflect tire construction and loaded operation; this calculation uses a geometric circle.

Sources & calculation method

Related guidance: NHTSA TireWise: tire sizes and safety. Accessed September 9, 2026. The equation above describes this tool’s model; the reference provides practical context and does not endorse this site.

Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.