Two ways of naming the same tire
Most passenger and SUV tires use metric sizes, such as 285/70R17: width in millimeters, aspect ratio as a percentage, and rim diameter in inches. Many off-road and light-truck tires use flotation or inch sizes, such as 33x12.50R15: overall diameter, section width and rim diameter, all in inches. Converting between the two helps you compare tires that are marketed differently, or find a metric tire that matches the height of an inch size.
Metric to inch
Example: 285/70R17. Sidewall = 285 × 0.70 = 199.5 mm = 7.85 in. Diameter = 2 × 7.85 + 17 = 32.7 in. Width = 285 ÷ 25.4 = 11.2 in. So 285/70R17 ≈ 32.7x11.2R17, which is why it is often sold and discussed as a "33-inch" tire.
Inch to metric
Going the other way is not one-to-one, because metric sizes come in fixed steps: widths end in 5 (265, 275, 285 and so on) and aspect ratios usually step by 5. The converter works out the inch size's width in millimeters and its aspect ratio, then searches every standard combination on the same rim and ranks them, favoring the closest diameter and then the closest width.
Example: 33x12.50R15. Width = 12.5 × 25.4 = 317.5 mm. Sidewall = (33 − 15) ÷ 2 = 9 in, so the aspect ratio is 9 ÷ 12.5 = 72%. The nearest metric dimensions are about 325/70R15 (32.9 in) and 305/75R15 (33.0 in). A 315/75R15 is slightly taller at 33.6 in.
The converter lists the mathematically closest sizes. Not every size is manufactured in every tread type and load range, so check what is actually available before you decide.
Rounding in inch sizes
Inch sizes are rounded, often generously. A tire sold as a "35" might measure 34.4 in, and a 285/75R17, which calculates to 33.8 in, is often described as a 34. When accuracy matters, for example for gearing or tight clearance, use the manufacturer's measured overall diameter for the exact tire model, not the nominal size.
Other things that change between sizes
- Load capacity: flotation LT tires and metric LT tires are rated differently from P-metric tires. Always match or exceed the load requirement on your door placard.
- Rim width range: inch sizes like 12.50 in wide tires typically call for wider wheels than a narrow metric size of the same height. Check the tire maker's approved rim widths.
- Speedometer: if you change diameter, see how far off it will be with the speedometer calculator.
To see all the dimensions for a size, use the tire size calculator. To compare two sizes to scale, use the tire size comparison.
Frequently asked questions
What is 285/70R17 in inches?
About 32.7 x 11.2 inches on a 17 inch rim, so roughly a 33x11.2R17. It is usually called a 33-inch tire.
What metric size equals 33x12.50R15?
There is no exact match. The closest metric dimensions are about 325/70R15 and 305/75R15, both close to 33 inches tall. A 315/75R15 is about 33.6 inches.
What metric size is a 35-inch tire on 17 inch wheels?
A 35x12.50R17 works out to about 317.5 mm wide with a 72% aspect ratio. The closest metric dimensions are around 325/70R17 or 305/75R17, both about 35 inches. 315/70R17 is about 34.4 inches.
Why does my 33-inch tire measure less than 33 inches?
Inch sizes are nominal and often rounded up. The measured diameter for a specific tire model is listed in the manufacturer’s specifications.
Can I convert sizes with a different rim diameter?
This converter keeps the same rim size. To compare tires on different rims, use the tire size comparison calculator.
Last reviewed November 2026. Spotted an error? Tell us.