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Unlike adult bikes, which are measured by the wheel size and the frame size, kids' bike sizes are usually determined only by the wheel size. The wheel sizes are usually measured in inches, and the most common kids' bike sizes are 12 inch (30.5 centimeters), 16 inch (40.6 centimeters), 20 inch (50.8 centimeters), and 24 inch (61 centimeters). Some larger children may even fit a 26 inch (66 centimeter) bike, which is considered an adult-sized bike; at that level, the size of the bike will also be measured according to the frame size. The smallest children will ride 12 inch bikes, which usually feature training wheels and may or may not feature pedals.
12 inch (30.5 centimeter) bikes are meant for children who have not yet learned to ride bikes on their own. They are low to the ground to accommodate the small stature of young children, and even though the kids will not be able to reach high or even moderate speeds, helmets should always be worn while riding. The next step up in kids' bike sizes is the 16 inch (40.6 centimeter) bike; 14 inch (35.6 centimeter) bikes do exist, but they are not nearly as common as other sizes. The larger wheel size will also translate into a slightly larger frame, and these bikes are also likely to feature training wheels to help kids learn to ride properly.
The next step up in kids' bike sizes is the 20 inch (50.8 centimeter) bike. This bike will feature larger wheels that are the same size as BMX wheels. The frame of such a bike is likely to be far smaller than a BMX frame but larger than a 16 inch (40.6 centimeter) kids' bike. By this time, kids can usually ride on two wheels, though it is still possible to put training wheels on this size of bicycle. Training wheels can also be installed on 24 inch (61 centimeter) bicycles, though again, they are not usually needed.
24 inch (61 centimeter) and 26 inch (66 centimeter) kids' bike sizes may or may not be considered exclusively kids' sizes at all. Many adults ride 24 inch (61 centimeter) bikes, and the vast majority of adult riders will fit a 26 inch (66 centimeter) bike. The frame sizes of these two types of bikes are also likely to change, allowing for a more accurate fit for larger children or adults.