The size of a tire can be found on the sidewall. This measurement is written as two numbers, separated by an X -- for example, 700X23 or 26X2.10. The first number is the diameter of the tire, in millimeters or inches. Most adult bikes come in either the 700mm size or the 26-inch size. You'll find 700 tires on road bikes and 26 tires on mountain bikes. The 700 tires follow the metric system because road bikes developed in Europe, while mountain biking, with it's 26-inch tire, began in the United States.
The second number on the tire's sidewall is it's width. On a 700X23 tire, the width of the tire is 23mm. As the width increases, the tire will have more surface contact with the ground. More surface contact gives you a more stable and comfortable ride. As the width decreases, the tire has less surface contact, and therefore less fiction, which makes your ride faster.
Width Fractions and Decimals
If you ride a bike with a 26-inch tire, the tire width will be denoted with either a decimal or a fraction -- for example, 26X1.75 or 26X1 3/4. Take note that even though the two tires appear to have the same width, if one is marked with a decimal and the other a fraction, they are actually two different sizes. If you're looking for an exact size match, make sure you don't choose one fraction tire and one decimal tire.
Measurement Discrepancies and the ISO
Measure the diameter and width of your tires, and you'll find that they don't exactly match what's written on the tire. The numerous sizing systems -- and companies that try to cheat on their sizes -- make it hard to know what the numbers on a tire really mean. Fortunately, the International Organization for Standardization, or ISO, has created a system for tire sizing to regulate tire measurement and take the guesswork out of choosing the proper tire. Most new tires and rims will show the ISO measurement. This measurement marks width first, followed by diameter. If you want an exact match when you replace an old tire, match the ISO number.
When it's time to buy new tires, always get tires with the same diameter as your old ones, but you can choose a different width. Your bike's rim will hold a range of tire widths, although not all widths are recommended for every tire. See the Resources for a tire width chart that shows proper compatibility.