Many new riders are confused by bike tire sizing. Hopefully the following tutorial by our Spin Doctor Tech Tip team will clear up some of the confusion.
Bike tires are sized by a simple combination of diameter and width. Modern road tires are sized in millimeters. The most common road tires are 700C’s, meaning they are about 700mm in outside diameter (O.D.). If the tire is 700 x 23 then it is ~700mm in diameter and the inflated tire is 23mm wide. MTB tires are sized in inches, so a mounted and inflated 26 x 1.95 tire is about 26” in O.D. and 1.95” wide.
Common road tire diameters:
700C is by far the most common modern size
650C is a smaller size used on some time trial and triathlon bikes
27 inch is a less common, older American size
27” & 700Cs are not interchangeable [see ISO Sizing below]
Common mountain bike tire diameters:
26″ is the most common size
29” is based on the 700C road size (see ISO Sizing below) and is essentially a wider and knobbier 700C tire fitted to a wider and stronger 700C rim Read more of this post