Tire size conversion calculator

Read a metric tire-size code and compare its nominal dimensions with another size.

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Tire size (width/aspect R rim)

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The size calculation uses nominal dimensions, not a measurement of an inflated tire. Enter only the size code, without a load index or speed rating.

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Updated 2026-09-19

How this conversion works

In 195/65R15, 195 is nominal width in millimeters, 65 is sidewall height as a percentage of width, and 15 is rim diameter in inches.

Nominal diameter = rim inches × 25.4 + 2 × width × aspect/100. Comparing nominal diameters gives a geometric difference, not a fit or replacement recommendation.

Next: Length converter

A worked example

195/65R15 has a calculated sidewall of 126.75 mm and nominal diameter of 634.5 mm, about 24.9803 inches.

Next: Speed converter

Before you use the result

Geometry only. Actual diameter, load, rim fit, clearance and manufacturer-approved sizes can differ. Follow the vehicle and tire manufacturer’s fitment requirements.

Next: Pressure converter

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