Truck and off-road tires are usually sold in inches — 33x12.50R15, 35x12.50R17 — while passenger cars use metric codes like 225/45R17. Both describe the same thing, but the numbers mean different things, which makes it hard to know whether a 315/70R17 is the same tire as a 35x12.50R17. This calculator converts in both directions so you can compare sizes, shop across brands and check fitment.
How to use the tire size conversion calculator
- Choose the direction. “Metric → inches” starts from a code like 265/70R17; “Inches → metric” starts from a code like 35x12.50R17.
- Enter the numbers from the sidewall, including the rim diameter.
- Read the results. The metric result shows the exact width and aspect ratio, and the nearest standard metric size.
For the full set of dimensions of a metric size use the tire size calculator; to find the speedometer effect of changing sizes use the speedometer error calculator.
Tire size conversion formula
Inches → metric: width (mm) = width (in) × 25.4; aspect = (diameter − rim) ÷ 2 ÷ width (in) × 100
- 25.4 — millimetres per inch
- Rim — identical in both systems
- Metric sizes are made in steps of 5 mm (width) and 5% (aspect), so the exact result is rounded to the nearest real size
Worked example: 35x12.50R17 to metric
- Width: 12.5 × 25.4 = 317.5 mm.
- Sidewall: (35 − 17) ÷ 2 = 9 in = 228.6 mm.
- Aspect ratio: 228.6 ÷ 317.5 = 72%.
- Nearest standard size: 317.5 → 315 mm, 72% → 70%, giving 315/70R17.
- Check: the metric size has diameter 17 + 2 × (315 × 0.70 ÷ 25.4) = 34.7 in — about 0.3 in less than the nominal 35.
Inch sizes in metric
Each row is calculated with the formulas above.
| Inch size | Width | Aspect ratio | Nearest metric size | Diameter |
|---|---|---|---|---|
| 31x10.50R15 | 266.7 mm | 76.2% | 265/75R15 | 787 mm |
| 33x12.50R15 | 317.5 mm | 72.0% | 315/70R15 | 838 mm |
| 33x12.50R17 | 317.5 mm | 64.0% | 315/65R17 | 838 mm |
| 35x12.50R17 | 317.5 mm | 72.0% | 315/70R17 | 889 mm |
| 35x12.50R20 | 317.5 mm | 60.0% | 315/60R20 | 889 mm |
| 37x12.50R17 | 317.5 mm | 80.0% | 315/80R17 | 940 mm |
Metric sizes in inches
| Metric size | Diameter | Width | Inch-style size (diameter rounded) |
|---|---|---|---|
| 225/45R17 | 24.97 in | 8.86 in | 25x8.86R17 |
| 235/65R17 | 29.03 in | 9.25 in | 29x9.25R17 |
| 245/75R16 | 30.47 in | 9.65 in | 30x9.65R16 |
| 265/70R17 | 31.61 in | 10.43 in | 32x10.43R17 |
| 285/75R16 | 32.83 in | 11.22 in | 33x11.22R16 |
| 315/70R17 | 34.36 in | 12.40 in | 34x12.40R17 |
Cautions when converting
A conversion gives sizes that are approximately equal, not identical. Metric sizes only exist in 5-unit steps, and tire makers measure their flotation sizes on a particular rim width, so the overall diameter of a “35-inch” tire can fall anywhere from about 34 to 35.5 inches. When fit matters — lifted trucks, tight wheel wells — look up the manufacturer's published overall diameter and width for the exact tire rather than relying on the nominal size.
Also remember that conversion only tells you the dimensions. Inch-sized tires are often LT (light truck) tires with a different load range, and an approximate metric twin may have a different load index or construction. If a size changes your overall diameter, the speedometer and gearing change too; see the tire size comparison calculator to measure the difference.
Sources
The inch-to-metric relationship (1 in = 25.4 mm exactly) and the metric size markings follow the ISO designation used by the Tire and Rim Association. The existence of sizes such as 35X12.50R17 and 31X10.50R15 is confirmed by the tire-size reference data collected from tiresize.com on 2026-10-09. Table values were computed from the formulas shown.
Frequently asked questions
How do I convert a metric tire size to inches?
Overall diameter = rim + 2 × (width × aspect ratio ÷ 100 ÷ 25.4); width in inches = width in mm ÷ 25.4. For 265/70R17 that gives about 31.6 in tall and 10.4 in wide — the 31x10.50R17 class.
How do I convert 35x12.50R17 to metric?
The width is 12.5 × 25.4 = 317.5 mm and the sidewall is (35 − 17) ÷ 2 = 9 in = 228.6 mm, so the aspect ratio is 72%. Metric tires come in steps of 5 mm and 5%, so the nearest size is 315/70R17.
Why is the metric result only “nearest”?
Metric sizes are made only in standard steps, so the exact aspect ratio is rounded to a size that exists. The exact diameter can differ by a few tenths of an inch.
Is a 35-inch tire exactly 35 inches tall?
It is the nominal diameter. Measured values vary by about ±1–2% with tire design, inflation, load and wear.
Does the rim size change in conversion?
No. The rim diameter is the same number in both systems — the R17 in 315/70R17 and 35x12.50R17 refers to the same 17-inch wheel.