Tire calculators are convenient but here's a trick (courtesy of John Griggs):

If you need to know a tire diameter and you don't have a resource like the internet available, you can use a conversion factor (1270) to simplify the calculation.

- Multiply the first two numbers in the tire size together
- Divide the result by 1270
- Add the third number to get the tire diameter in inches

Examples:

__285/75R16:__
(285 x 75 / 1270) + 16 = 32.8" diameter

__235/75R15:__
(235 x 75 / 1270) + 15 = 28.9" diameter

__325/35VR19:__
(325 x 35 / 1270) + 19 = 28.0" diameter

The above will work on any tire that is sized by three numbers in the commonly used metric/English format like 235/75R15, for example. The format may be called P-Metric, Euro-Metric, LT-Metric, etc.

[Note that the 1270 conversion factor cannot be used for “inch size” flotation tires which have the

*approximate* (but usually slightly exaggerated) diameter embossed right on the sidewall (like 31x10.50R15 or 33x12.50R15). If you need a more accurate diameter spec for these tires, consult the manufacturer's literature or website.]