Almost all bicycles use pneumatic tires. These are tires filled with pressurized air to maintain their shape and absorb road vibration. There are a number of different tire sizes still in use. The tire size, and, by extension, the tube size, depends on the type of bike, year of production and location of production, but there’s no need to know all of this information.
Look at the side of the tire, where there are quite a few numbers. The numbers mentioning "psi" (pounds per square inch) and "bars" refer to pressure. Ignore them when sizing your tire.
Look for the numbers referring to the size, such as 26x1.75 or 700x28c. Usually, these numbers will be all you need, but there are some odd-sized wheels for older bikes or specialized bikes. If you want to avoid mistakes, look for the International Organization for Standardization (ISO) number. It will be next to the sizes mentioned above and will contain two numbers, a dash, then three numbers (xx-xxx). These numbers are the tire's size in millimeters.
Find the tire width. The width is the first two numbers in the ##-### format. You can get away with a tube that's a little smaller than the tire, but you don't want to use a wider tube. A wider tube will increase the likelihood of a pinch flat when installing it in the tire.
Use the correct valve stem. There are two types of valves stems most commonly used: Schrader and Presta. Schrader is thicker than Presta, and if your wheel is made for a Presta valve, a Schrader won't fit. Conversely, a Presta valve in a Schrader-sized-hole might lead to a flat, though many people do it without any problem.