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Tire Size Calculator

Enter a metric tire size to get its true overall diameter, sidewall height, circumference, and revs per mile — the numbers your speedometer and odometer actually respond to.

mm
Section width in millimetres, e.g. 215
%
Sidewall height as % of width, e.g. 60
in
Rim diameter in inches, e.g. 16
Overall diameter26.16 in
Diameter (metric)664 mm
Sidewall height5.08 in
Circumference82.18 in
Revs per mile771
diameter (mm) = rim × 25.4 + 2 × width × aspect ÷ 100  ·  revs/mile = 1,609,344 ÷ circumference
Paper size vs. real size: the number above is the nominal spec-sheet diameter. Real-world diameter varies with tread depth and manufacturer — typically ±0.2–0.4 in between two tires of the same size. That is why fit decisions should keep margin around the thresholds, not ride exactly on them.
Before you buy, also check: load index and speed rating ≥ stock · bolt pattern and centre bore fit your wheels · brake caliper clearance (spoke shape) · full-lock suspension clearance. A diameter match does not guarantee a fit.

Assumptions & sources

Diameter formularim×25.4 + 2×(width×aspect/100)Standard tire geometry (same formula used by tiresize.com & crawlpedia)
Revs per mile constant1 mile = 1,609,344 mmExact imperial-metric conversion
Real diameter variance±0.2–0.4 in by brand/treadCommunity reports, r/Justrolledintotheshop sample

Frequently asked questions

What does 215/60R16 actually mean?
215 is the section width in millimetres, 60 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 16 is the wheel diameter in inches. Sidewall height = 215 × 0.60 = 129 mm (5.08 in).
Why do revs per mile matter?
Your speedometer and odometer count driveshaft or wheel revolutions. A tire with fewer revs per mile travels further per revolution, so the speedometer reads low. Matching revs per mile is what keeps the speedometer honest after a tire change.
Is a taller tire bad for my car?
Slightly taller (within about 3% diameter) usually only shifts the speedometer a little. Beyond about 5% you risk rubbing at full lock, raised gearing that hurts acceleration, and inaccurate odometer readings. The safe range depends on your drivetrain — see our tire size difference guide.
Does the calculated diameter match the real tire?
Not exactly. The spec diameter assumes nominal dimensions; real tires vary by manufacturer and tread depth, typically ±0.2–0.4 in. Use the calculator for comparisons and keep margin near thresholds.