Automotive calculator

Tire Size Calculator

Convert a metric tire size into estimated sidewall height, overall diameter, circumference, and revolutions per mile.

Read a metric tire size

A size such as 225/65R17 contains three dimensions: a nominal 225 mm section width, a sidewall height that is 65% of that width, and a 17-inch wheel diameter. The calculator converts the sidewall to inches and estimates unloaded diameter and circumference.

Worked example: 225/65R17

The sidewall is 146.25 mm, or about 5.76 inches. Adding two sidewalls to the 17-inch rim produces an estimated 28.52-inch diameter. The circumference is about 89.59 inches, equal to roughly 707 revolutions per mile.

What the estimate does not prove

• A calculated diameter does not confirm load capacity, speed rating, wheel width, clearance, or safe fitment.
• Actual mounted dimensions vary by manufacturer, tread, rim width, inflation, load, and wear.
• Changing diameter can affect speedometer readings, gearing, braking systems, and clearance.
• Use the tire information placard and owner’s manual for the vehicle manufacturer’s specified size and pressure.

Safety note: NHTSA advises consumers to use the tire size recommended by the vehicle manufacturer. Confirm any substitution with a qualified tire professional.

Frequently asked questions

What does the R mean?

R identifies radial construction. It separates the aspect-ratio portion from the wheel-diameter portion; it is not a radius measurement.

Is calculated diameter the same as measured diameter?

No. This is a nominal geometric estimate. A tire maker’s published specification or a careful real-world measurement is more appropriate when small clearance differences matter.

Why calculate revolutions per mile?

It helps compare rolling distance between nominal sizes. Vehicle systems may be calibrated to a specific range, so it should not be used alone to approve a tire change.

How to get a dependable result

• Use measurements from the same trip, billing period, or planning scenario.
• Keep units consistent and enter positive numbers unless the field explicitly allows zero.
• Treat the result as an estimate. Real-world conditions, rounding, equipment efficiency, and changing prices can affect the final amount.

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Sources and review notes

Tire terminology and replacement-safety guidance are based on the National Highway Traffic Safety Administration’s TireWise guidance.

Last reviewed September 2026. This calculator provides an estimate for planning and education; it is not a substitute for a manufacturer specification, professional inspection, or safety advice.