What the calculator reads from the size
A metric size like 265/70R17 gives the three numbers the maths needs: the section width in millimetres (265), the aspect ratio, which is the sidewall height as a percentage of the width (70), and the rim diameter in inches (17). A flotation size like 33x12.50R15 gives the overall diameter, the width and the rim in inches directly. Letters such as P or LT and the load and speed marks after the size do not change the dimensions. For every other marking on the sidewall, see how to read a tire sidewall.
The formula
For a metric size, the sidewall is the width times the aspect ratio. Two sidewalls plus the rim give the overall diameter:
sidewall (mm) = width × aspect ÷ 100
diameter (in) = rim + 2 × sidewall ÷ 25.4
circumference = π × diameter
revolutions per mile = 63,360 ÷ circumference (in)Worked example, 265/70R17: sidewall = 265 × 0.70 = 185.5 mm = 7.30 in. Diameter = 17 + 2 × 7.30 = 31.61 in (803 mm). Circumference = 99.3 in, so the tire turns about 638 times per mile.
Why the real tire can measure differently
The size code is a nominal size. Two tires with the same code from different brands can differ in diameter, and an all-terrain or mud-terrain tread often adds height. Rim width changes the section width too. When clearance is tight, check the manufacturer's published dimensions for the exact tire.
Questions
How tall is a 265/70R17 tire?
About 31.6 inches (803 mm). The sidewall is 265 × 0.70 = 185.5 mm, and 17 + 2 × 7.30 = 31.61 inches.
How do I convert a metric tire size to inches?
Multiply the width by the aspect ratio to get the sidewall in mm, divide by 25.4 to get inches, double it and add the rim diameter. The calculator above does this for any size and also shows the flotation equivalent, like 31.6x10.4R17.
Is a 285/75R16 a 33-inch tire?
Close to it: the nominal diameter is 32.8 inches, which is why it is often called a metric 33-inch tire.
What does the aspect ratio mean?
It is the sidewall height as a percentage of the tire width. A 70 aspect ratio on a 265 mm tire gives a 185.5 mm sidewall. A lower number means a shorter sidewall.