Metric Tire Size Conversion
Metric in, inches out. The conversion is three steps.
To convert a metric tire size to inches: multiply width by aspect ratio and divide by 25.4 for sidewall inches, then add twice the sidewall to the rim diameter for overall diameter. For 265/70R17, the sidewall is 265 x 0.70 / 25.4 = 7.3 inches and the diameter is 17 + 14.6 = 31.6 inches. Circumference is pi times diameter, about 99.3 inches.
Step 1: sidewall in inches
Sidewall millimeters = width x aspect / 100. For 265/70R17: 265 x 0.70 = 185.5 mm. Divide by 25.4 mm per inch: 7.3 inches.
This is the number most people never compute, but it is the bridge between the metric width and the imperial diameter.
Step 2: overall diameter
Diameter = rim + 2 x sidewall. For our example: 17 + 2 x 7.3 = 31.6 inches. That single number is what the speedometer, gearing, and clearance all care about.
Sanity check against known sizes: a 265/70R17 at 31.6 inches is a classic light-truck size, which matches real-world spec sheets.
Step 3: circumference and revs
Circumference = pi x diameter = about 99.3 inches. Revs per mile = 63,360 / circumference = about 638.
Tire makers publish revs per mile for exact fitment math; the formula gets you within a percent, close enough for comparison shopping.
Skip the arithmetic
Convert any size on the calculator.
Metric conversion questions
How do you convert 265/70R17 to inches?
Sidewall: 265 x 0.70 / 25.4 = 7.3 inches. Diameter: 17 + 2 x 7.3 = 31.6 inches. Circumference: pi x 31.6 = 99.3 inches.
Why are tire sizes metric and imperial mixed?
Tread width has been metric for decades while rim diameters stayed imperial from the early wheel standards. The mixed format is entrenched globally, so conversion is a permanent skill.
What is 25.4 in tire math?
One inch equals exactly 25.4 millimeters. Dividing metric millimeters by 25.4 converts any tire dimension to inches.