The National Tire & Wheel Tire Calculator will calculate the tire height and width of a Metric Size Tire for you and convert it to the easier to understand good old American inches.
Using the P-Metric Style box, enter the WIDTH in the first box . (This number represents the tread width measured in millimeters.)
Enter the ASPECT RATIO into the second box. (This number represents the aspect ratio of the sidewall versus the tread width displayed as a percentage.)
Enter the DIAMETER into the third box. (This number represents your wheel size.)
Example: To convert a 285/75/R16 metric sized tire you would enter 285 in the far left field; 75 in the middle field; 16 in the field after the R. Then you would Click the Convert P-Metric to inches button.
If you need to go the other way and convert the standard size to metric, simply start with the box on the right that says "Inch Style" and work backwards.