Check the tires you are replacing and find the size of the tire which is typically molded or printed on the sidewall of the tire. The format of this label can vary between the Traditional English sizing (eg. 26x2.00 or 29x2.20), the French sizing (eg. 700x28c, 650x45) or the ETRTO (eg. 28-622, 45-584). All three of these sizing formats give the same information, diameter and width.
The ISO (International Organization for Standardization) has developed a universal tire and rim sizing system that help with determining the size of tire you need. English size (eg. 26x2.00) and French size (eg. 700x28c) tire dimensions are still used today but most bicycle tires are also marked according to ETRTO or European Tire and Rim Technical Organization (eg. 28-622)
The ISO system uses two sets of numbers to determine tire size. The first number indicates the width and the second indicates the inner bead diameter of the tire. The ETRTO size specification 28-622 would indicate a tire width of 28mm and the tire's inner diameter of 622mm. Both the ISO as well as either the English or French size will be listed on the sidewall on the hot patch or directly molded into the rubber of the tire.
While a tire's diameter needs to be an exact match to the rims on your bicycle, there is some choice on what width tires you can use. Some like to use wider tires for more grip off-road or a more comfortable ride where others may opt for a narrower tire for more speed and efficiency on pavement. However, it is important to make sure that your bicycles frame or brakes have clearance to ensure that the tire does not rub the bicycle during rotation and that you have a little bit of clearance on either side of the tire for dirt or debris (we recommend around 3mm of clearance between the tire and any other parts of the bicycle).