How to convert tire sizes
Metric to inches
Diameter (in) = rim + 2 × (width × aspect ÷ 100) ÷ 25.4 · Width (in) = width ÷ 25.4
Example: 265/70R17 → sidewall 185.5 mm = 7.30 in → diameter 17 + 2 × 7.30 = 31.6 in; width 265 ÷ 25.4 = 10.4 in. So 265/70R17 ≈ 31.6x10.4R17.
Flotation to metric
Width (mm) = width (in) × 25.4 · Aspect ratio = ((diameter − rim) ÷ 2) ÷ width × 100
Example: 33x12.50R15 → width 317.5 mm, sidewall 9 in, aspect 9 ÷ 12.5 = 72 %. The nearest real sizes are 325/70R15 (32.9 in) and 315/75R15 (33.6 in).
Flotation sizes and their closest metric size
| Flotation | Closest metric | Metric diameter | Difference |
|---|---|---|---|
| 31x10.50R15 | 265/75R15 | 30.6 in | −1.1% |
| 33x12.50R15 | 325/70R15 | 32.9 in | −0.3% |
| 33x12.50R17 | 315/65R17 | 33.1 in | +0.4% |
| 33x12.50R18 | 315/60R18 | 32.9 in | −0.4% |
| 33x12.50R20 | 325/50R20 | 32.8 in | −0.6% |
| 35x12.50R17 | 325/70R17 | 34.9 in | −0.2% |
| 35x12.50R18 | 315/70R18 | 35.4 in | +1.0% |
| 35x12.50R20 | 315/60R20 | 34.9 in | −0.3% |
| 37x12.50R17 | 315/80R17 | 36.8 in | −0.4% |
Popular truck sizes in inches
| Metric | Inches | Usually called |
|---|---|---|
| 235/75R15 | 28.9x9.3R15 | 28.9″ ≈ 29-inch tire |
| 245/75R16 | 30.5x9.6R16 | 30.5″ ≈ 30-inch tire |
| 265/75R16 | 31.6x10.4R16 | 31.6″ ≈ 32-inch tire |
| 265/70R17 | 31.6x10.4R17 | 31.6″ ≈ 32-inch tire |
| 285/70R17 | 32.7x11.2R17 | 32.7″ ≈ 33-inch tire |
| 275/65R18 | 32.1x10.8R18 | 32.1″ ≈ 32-inch tire |
| 275/60R20 | 33.0x10.8R20 | 33.0″ ≈ 33-inch tire |
| 285/55R20 | 32.3x11.2R20 | 32.3″ ≈ 32-inch tire |
Frequently asked questions
What does 33x12.50R15 mean?
It is a flotation (inch) size: about 33 inches tall, 12.50 inches wide, for a 15-inch wheel. Flotation sizes are common on off-road and light-truck tires in North America.
Is a 285/70R17 the same as a 33-inch tire?
Nearly. 285/70R17 works out to 32.7 inches tall and 11.2 inches wide, which is why it is sold as a "33-inch" metric alternative. Real tires vary by a few tenths of an inch between brands.
Why is there no exact metric equivalent?
Metric sizes come in 10 mm width steps and 5 % aspect-ratio steps, so most flotation sizes fall between two metric sizes. The converter lists the nearest real sizes and how far their diameter is from the original.
How do I convert millimetres to inches for tires?
Divide by 25.4. The section width of 265 mm is 265 ÷ 25.4 = 10.43 inches. For the overall height add the rim diameter to twice the sidewall (width × aspect ratio).