View Here : Tire Size Meaning
Some tires may display the tire size in the International Organization for Standardization, or ISO, system. In this case, both the diameter and width are represented in millimeters. An example is a 700 x 20c tire would be expressed as 20-622.
There are, however, online tire size calculators that help you compute whether the tires have similar overall diameters. The rule is, if you want to change your tire size, they should not have a difference in diameter of more than 1%, ideally, it should be less than that.