Calculating tire size is a rather simple concept. Sure its easy to get on the internet real quick and find the answer, but sometimes you dont always have it on hand. Tires that have metric sizing can look confusing to read, but in actuality it just takes a few simple calculations. 265/70R17 is a common tire size. what this means is 265=width of tire in mm + 70= percentage of width (finds sidewall height) + R17= rim size in inches.
By taking this information we find the tire size by taking 256mm (10.43") + 185.5mm (7.301"x2..we double this measurement to account for top and bottom part of sidewall) + 17" = 31.6" overall tire size.
mm to inch conversion= 25.4mm is equivalent to 1"
I hope i do not confuse anyone with this, i tried to make it as straight forward as possible.
Tire size calculator