How to use the metric tire conversion
- Enter the section width in millimeters (the first number, e.g. 265).
- Enter the aspect ratio as a whole-number percentage (the second number, e.g. 70).
- Enter the rim diameter in inches (the number after the R, e.g. 17).
- Read the converted dimensions in inches below the inputs.
How metric tire sizes work
A metric tire size such as 265/70R17 contains three measurements. The first number (265) is the section width in millimeters, measured from sidewall to sidewall when mounted on the correct rim. The second number (70) is the aspect ratio, which means the sidewall height is 70 percent of the section width. The letter R indicates radial construction, and the final number (17) is the rim diameter in inches.
Because the size mixes millimeters and inches, a metric tyre conversion is needed to express every dimension in a single unit. The standard conversion factor is 1 inch = 25.4 mm exactly.
Formulas used for the conversion
The math behind this tire size calculator involves four steps:
- Section width in inches: divide the width in mm by 25.4.
- Sidewall height in inches: multiply the width in mm by the aspect ratio divided by 100, then divide by 25.4.
- Overall diameter: add two sidewall heights to the rim diameter.
- Circumference: multiply the overall diameter by pi (3.14159265...).
For a 265/70R17 tire: the section width is 265 / 25.4 = 10.43 inches. The sidewall height is 265 x 0.70 / 25.4 = 7.30 inches. The overall diameter is 17 + (2 x 7.30) = 31.61 inches. The circumference is 31.61 x 3.1416 = 99.30 inches.
Common metric tire sizes in inches
This reference table shows the results of the metric tyre conversion for several popular sizes. Values are rounded to two decimal places.
| Metric size | Width (in) | Sidewall (in) | Diameter (in) |
|---|---|---|---|
| 205/55R16 | 8.07 | 4.44 | 24.88 |
| 225/45R17 | 8.86 | 3.99 | 24.97 |
| 265/70R17 | 10.43 | 7.30 | 31.61 |
| 285/75R16 | 11.22 | 8.42 | 32.83 |
| 315/75R16 | 12.40 | 9.30 | 34.60 |
Why overall diameter matters
The overall tire diameter affects speedometer accuracy, gearing and vehicle clearance. When you change tire sizes, even a small difference in diameter alters the effective gear ratio. A larger diameter means the vehicle travels farther per wheel revolution, causing the speedometer to read lower than the actual speed. Conversely, a smaller diameter makes the speedometer read high.
Most manufacturers recommend staying within 3 percent of the original overall diameter to avoid issues with anti-lock brakes, traction control and transmission shift points. Use this tool to compare the overall diameter of a new tire against the factory size before purchasing.
Common mistakes when converting tire sizes
One frequent error is treating the aspect ratio as a fixed measurement in millimeters rather than a percentage. An aspect ratio of 70 does not mean 70 mm; it means 70 percent of the section width. Another mistake is forgetting that the overall diameter includes two sidewalls (top and bottom), not just one.
Some people also confuse the section width with the tread width. The section width is measured at the widest point of the sidewalls, while the tread width is the flat contact area, which is always narrower. Finally, mixing up the exact conversion factor matters: use 25.4 mm per inch, not an approximation like 25 mm.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Common truck tire (265/70R17) | Section width (mm): 265; Aspect ratio (%): 70; Rim diameter (inches): 17 | Section width: 10.43 in; Sidewall height: 7.30 in; Overall diameter: 31.61 in; Circumference: 99.29 in; Revolutions per mile: 638.1 |
| Passenger car (205/55R16) | Section width (mm): 205; Aspect ratio (%): 55; Rim diameter (inches): 16 | Section width: 8.07 in; Sidewall height: 4.44 in; Overall diameter: 24.88 in; Circumference: 78.16 in; Revolutions per mile: 810.7 |
| Large off-road (315/75R16) | Section width (mm): 315; Aspect ratio (%): 75; Rim diameter (inches): 16 | Section width: 12.40 in; Sidewall height: 9.30 in; Overall diameter: 34.60 in; Circumference: 108.71 in; Revolutions per mile: 582.9 |
| Low-profile sport (245/40R18) | Section width (mm): 245; Aspect ratio (%): 40; Rim diameter (inches): 18 | Section width: 9.65 in; Sidewall height: 3.86 in; Overall diameter: 25.72 in; Circumference: 80.79 in; Revolutions per mile: 784.2 |
FAQ
What does 265/70R17 mean?
It means the tire has a section width of 265 mm, a sidewall height equal to 70 percent of that width, radial construction, and fits a 17-inch rim.
How do I convert tire width from mm to inches?
Divide the width in millimeters by 25.4. For example, 265 mm divided by 25.4 equals approximately 10.43 inches.
Is the aspect ratio a measurement in millimeters?
No. The aspect ratio is a percentage. A ratio of 70 means the sidewall height is 70 percent of the section width, not 70 mm.
How much can I change my tire diameter safely?
Most guidelines suggest staying within 3 percent of the original overall diameter to keep speedometer readings, ABS and traction control working correctly.
Does this tool store or send my data?
No. The metric tyre conversion runs entirely in your browser. Nothing is uploaded to any server.
Can I use this for motorcycle tires?
Yes. Motorcycle tires use the same metric format (width/ratio-rim), so the formulas apply identically.