Guide to Kid Bike Sizes: Children's bike sizes are determined by wheel diameter, not seat height and frame size as is the case with adult bicycles. The chart below should help you narrow your search, but it is not a substitute for an expert helping you find a bike that is most suitable.  A critical factor is the brakes.  Coaster brakes tend to be easier for young kids to use but they become less common as the bikes get bigger.  It can be a conundrum and dangerous when kids who are tall for their age fit bigger bikes with hand brakes that they can't operate well.  After kids outgrow children's bikes they'll move into small-framed adult bikes with 26 inch, 27 inch or 700c wheels, which are sized by the length of the seat tube.

* The size doesn't have any relationship to a real measure: It is certainly not the diameter of the rim. Generally the given size is closer to the tire/tyre diameter from tread-to-tread, but it is usually greater that this distance as well.