Tire size calculator and decoder
Type a size and see what each part means.
Every part of a tire size code, decoded, plus the physical dimensions that decide whether it fits: overall diameter, section width, sidewall height, circumference and revolutions per mile.
Which number is which?
A metric tire size reads like a code. Take 205/55R16 91V:
| Part | Meaning | In this example |
|---|---|---|
| 205 | Section width in millimetres, measured at the widest point of the inflated tire | 205 mm (8.07 in) |
| 55 | Aspect ratio: sidewall height as a percentage of section width | 112.75 mm (4.44 in) |
| R | Radial construction. Nearly every passenger tire made today | Radial |
| 16 | Wheel (rim) diameter in inches | 16 in |
| 91 | Load index: maximum load the tire can carry at full pressure | 1,356 lb per tire |
| V | Speed rating: maximum sustained speed | 149 mph |
From code to physical size
The three numbers that decide fitment are overall diameter, section width and rim diameter. Overall diameter comes from the rim plus twice the sidewall: for 205/55R16, the sidewall is 205 × 0.55 = 112.75 mm, so the tire stands 16 in + 2 × 4.44 in = 24.88 in tall.
Revolutions per mile follows directly from diameter: 63,360 inches per mile divided by the circumference. A 24.88-inch tire turns about 811 times per mile, and that number is what your speedometer, odometer and transmission calibration assume.
Flotation sizes
Light truck tires are often sold in inches instead: 33X12.50R15 is 33 inches tall, 12.5 inches wide, on a 15-inch wheel. The calculator accepts both formats so you can compare a metric size against a flotation size directly.