Since children come in many sizes, the proper bike size for a child depends on the child's age and height. Children's bike sizes are usually measured by tire diameter.
Bikes with 12-inch wheels are recommended for toddlers and preschool-aged children, while bikes with 24-inch wheels are recommended for youths over 12. However, children that are tall or short for their age may need a larger or smaller bike. EBicycles.com features a bike size calculator to help find the right tire and crank size for a child. Visitors can input the age and gender of the child, and the calculator recommends a bike size and notes information about the average height for that age.