Common Tire Sizes
Most tire sizes are specified with two numbers. The first number refers to the outside diameter of the tire, and the second number refers to the width. For example, a standard road tire is specified as 700C x 23, which indicates a tire that is approximately 700mm in diameter and 23mm wide. In actuality, neither of these number is an exact measurement.
There are five tire diameter sizes that are in common use on modern standard adult bicycles.
- 26" is the most commonly used size for modern mountain bikes and is also used by many hybrid bikes.
- 27" is an older size that was used for road bikes, but since so many bikes were built with this size, tires of this size are still readily available.
- 29" is actually the same rim diameter as 700C, although most 29" tires will not fit 700C road rims because they're too wide. 29" tires are used for large-wheeled mountain bikes.
- 650C is used for smaller road bikes and triathlon bikes.
- 700C is the most commonly used size for modern road bikes.
The second number in the tire size refers to the width of the tire. However, these numbers are often slightly exaggerated, so a 700x28 tire may actually be only 26mm wide. This is probably the result of competitive pressures since the narrower tire will be lighter, and light weight is the driving force behind many buying decisions in the cycling industry.
This would all seem relatively straightforward except that older bicycles often used some of these same size designations (particularly 26" and 27") for tires that are not compatible with these sizes. If you have a bike that is more than about 15-20 years old, please read the section below on older tire sizes.