Children's bikes are traditionally measured by wheel size in inches.
The sizes below are a guide as to what wheel size would be suitable
for a particular age group
N.B All BMX bikes come with a 20" wheel and a frame size from 10" to 12" frames do vary depending on the manufacturer).
BMX bikes are suitable for ages form 8 years to mid teens
Adult bikes are measured by the size of the frame(measured in inches form the center of the bottom bracket to the top of the seat tube).
The sizes below are a guide as to which frame size would be suitable for the rider's height.
Road bikes are also measured by the frame size as above but the size is listed in centimeters rather than inches.