Tire size calculator
One size at a time: type a size code and this page decodes every part of it and works out the diameter, sidewall, circumference and revolutions per mile.
| Part of the code | Value | Meaning |
|---|
Worked-out dimensions
| Dimension | Metric | Imperial |
|---|
The short answer
A metric tire size like 265/70R17 packs four facts into one string: the tread is 265 mm wide, the sidewall is 70% of that width (185.5 mm), the construction is radial (R), and it fits a 17-inch wheel. From those, the overall diameter is 17 × 25.4 + 2 × 185.5 = 802.8 mm (31.61 in), the circumference is π × 802.8 ≈ 2,522 mm, and the tire turns about 637 times per mile. That diameter figure is the one that decides whether a different size can replace it — the comparison calculator puts any two sizes side by side.
Common sizes, decoded
| Size | Diameter | Width | Sidewall | Revs / mile |
|---|---|---|---|---|
| 265/70R17 | 802.8 mm (31.61 in) | 265 mm | 185.5 mm | 2005 |
| 205/55R16 | 631.9 mm (24.88 in) | 205 mm | 112.8 mm | 2547 |
| 225/65R17 | 724.3 mm (28.52 in) | 225 mm | 146.3 mm | 2222 |
| 31x10.50R15 | 787.4 mm (31.00 in) | 267 mm | 203.2 mm | 2044 |
Calculated from the size codes with the standard formulas on this page. Manufacturers' actual measurements vary slightly by brand and model; the size code is the reference, not a tape measure.
Frequently asked questions
What if my tire starts with P or LT?
The prefix states the service type — P for passenger, LT for light truck — and does not change the dimensions. The calculator accepts sizes with or without the prefix. Load index and speed rating (the "115T" after the size) describe capacity and speed capability, not size, and are covered in our reading guide.
How do flotation sizes like 35x12.50R17 work?
Flotation sizes state the answer directly: 35 is the overall diameter in inches, 12.50 is the width in inches, and R17 is the wheel. The sidewall is (35 − 17) ÷ 2 = 9 inches. The calculator reads them and converts everything to the same figures as metric sizes.
Why is my measured diameter slightly different from the calculated one?
The size code is nominal. Brands round differently, tread depth adds a few millimetres when new and removes them as the tire wears, and pressure and load change the effective rolling diameter. A small difference between the tape and the formula is normal.