Tire Size Calculator

How is tire diameter calculated from the size code?

A tire size like 225/45R17 means the tread width is 225mm, the sidewall height is 45% of that width (the aspect ratio), and the wheel diameter is 17 inches. Overall diameter = wheel diameter + 2 x sidewall height. Example: 225/45R17 has a sidewall of 225 x 0.45 = 101.25mm, so the overall diameter is 17 x 25.4 + 2 x 101.25 = 634.3mm, about 25.0 inches.

How do I read and calculate a tire size?

A tire size like 225/45R17 encodes tread width in mm, sidewall aspect ratio as a percent of that width, and wheel diameter in inches. Multiply width by aspect ratio for sidewall height, then add twice the sidewall to the wheel diameter for overall diameter. Example: 225/45R17 has a sidewall of 101.25mm and an overall diameter of about 634.3mm (24.97 in).

Steps to calculate tire size

1. Enter the tire size code exactly as printed on the sidewall, such as 225/45R17 (width in mm / aspect ratio in % / R for radial / wheel diameter in inches).
2. The calculator parses the three numbers and computes sidewall height as width x (aspect ratio / 100).
3. Overall diameter is the wheel diameter (converted to mm) plus two sidewalls, since the sidewall appears above and below the wheel.
4. Circumference is pi x diameter, which is then used to compute how many full rotations the tire makes per kilometer and per mile.
5. To compare two sizes, enter a second size — the calculator shows the percentage difference in diameter and, since a speedometer is calibrated to the original tire, how far off the reading becomes at common speeds with the new tire fitted.

Formula

• Width = tread width in millimeters, the first number in the size code
• Aspect ratio = sidewall height as a percent of width, the number after the slash
• Wheel diameter = rim diameter in inches, the number after 'R'
• Original / New diameter = overall diameter of the tire the speedometer was calibrated to vs. the replacement tire

Example tire size calculations

Frequently asked questions

Why does changing tire diameter throw off my speedometer?

A speedometer measures wheel rotation speed and converts it to a road speed using the diameter of the original tire. A larger-diameter tire covers more distance per rotation, so the car is actually going faster than the speedometer shows; a smaller tire means the car is going slower than indicated.

What does the R in a tire size mean?

R stands for radial, the standard tire construction used on almost all passenger vehicles today, where the fabric plies run radially (perpendicular to the direction of travel) rather than diagonally as in older bias-ply tires.

How much diameter difference is considered safe when changing tire size?

Many tire professionals suggest keeping overall diameter within about 3% of the original size to avoid speedometer error, clearance issues, and strain on the drivetrain, though the acceptable range can depend on the specific vehicle.

Does this calculator account for tire wear or load rating?

No. This calculator only computes geometry from the size code — width, aspect ratio and wheel diameter. It does not account for tread wear (which slightly reduces effective diameter over the tire's life), load index, or speed rating.

This calculator computes tire geometry directly from the standard size code using nominal manufacturer dimensions. Actual measured diameter can vary slightly by brand and tire model, and speedometer error in a real vehicle can also depend on factors the manufacturer calibrated for, so treat results as close estimates rather than exact figures.