Bike tire tubes are forgiving. It's important to get the diameter right, but tubes are also measured in widths. Widths can be altered slightly because tubes stretch to fit the available space inside the tire. Some tubes are labelled one-size-fits-all widths, but it's best to start out with the right size if you have a choice. It's easy enough if you use the information printed on the side of every tire, and every tube.
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Read the information printed on one side of the tire. There will be two sets of numbers. For mountain bikes, kids bikes, cruisers and BMX bikes the numbers will be in inches. For example, a typical mountain bike tire will have 26-by-2.00 printed on the side. Buy a tube that has those numbers marked on it; look for the number 26 first, and then select the width, 2.00.
Read the side of a road bike tire. They also have two sets of numbers, but they are measured in centimetres. For example, a typical road bike tire will have 700-by-23 marked on the side. The first number is the diameter and the second is the width. Buy tubes that have those numbers marked on it. Look for 700 first, and then select the width.
Read the tube boxes if you can't find the right size. Often, the tube boxes will have tubes that fit several different widths. The first number must be right, but the second set of number can have a wide range spanning from low numbers to high numbers. For example, a mountain bike tube might say, "fits 26-by-1.92, through 2.125." This tube is a universal width and will fit your mountain bike.
Read the box on a road bike tube. It might say, "fits 700-by-23 through 28." This is a universal width road tire and will fit your road bike.