The best way to choose the right tire size for a bicycle is to look at the side of the present tire. Other places to find the right size is the paperwork that came with the bike, the manufacturer's website, or simply measure the diameter and width of the wheel.
Most adult bikes, such as mountain bikes, have a 26 or 29-inch diameter tire. Standard road and racing bike sizes are 650 or 700 millimeters in diameter. Width also comes into play when choosing a bicycle tire, and the width is the second number in the tire size. For instance, if looking at a mountain bike tire, a person may see 26 x 1.5. This means the tire is 26 inches in diameter and 1.5 inches wide.