The diameter of your tire is written on your tire's sidewall. It's the first number in the number set that's separate with an X. For example, a 26x2.1 tire has an diameter of roughly 26 inches.
The width of the tire is the second number in the number set. For example, a 700X23 tire has a width of 23MM.
Inches and Millimeters
Adult bike tires are measured using either inches or millimeters. The standard size tire for a mountain bike is 26 inches and the standard for a road bike is 700MM.
Fractions and Decimals
The width of a tire may be written as either a fraction or a decimal. Unfortunately, a 1 1/4" width is not the same size as a 1.25 width, even though they have the same numeric value.
Not True to Measurement
If you measure the width of your 25MM tire, you may find that it is actually 24MM. Companies have been able to cheat by marking a 24MM tire as a 25MM tire, thereby making their "25MM" tire lighter than their competitors.
The diameter of your new tire must match the diameter of your wheel. Every wheel can accommodate a range of tire widths. See the Resource section for a compatibility chart.
Article Written By Kathrine Cole
Kathrine Cole is a professional outdoor educator. She teaches rock climbing, backpacking, cycling, and bike maintenance classes. She is a graduate of the National Outdoor Leadership School, a Wilderness First Responder, and a Leave No Trace Trainer.