Bicycles in children's sizes are categorised according to wheel size, rather than frame size. Unlike clothes, your child should not "grow into" a bicycle. Selecting a bike that is too big might be dangerous for your child. Before going to the bike shop, inform your child of the bike size that will fit him. This may help ward off protests if he prefers a certain colour of bike in the wrong size.
Measure the inside of your child's leg from top to bottom. Then measure your child's total height.
Compare the wheel measurements of the bikes to your child's measurements. If your child has an inseam measurement of at least 18 inches, with a total height of at least 41 to 49 inches, then a 16-inch wheel size is appropriate. Wheels that are 20 inches are for kids at least 50 to 56 inches tall, with an inseam of at least 23 inches. The minimum inseam measurement for a 24-inch wheel is 26 inches, with a total height of at least 57 inches.
Seat your child on a few different bikes with the correct measurements to test them out. When your child sits on the seat properly, he should be able to touch the ground with the balls of his feet. If the bike passes this test, have your child stand up, straddling the bar on the top of the bike frame. Both feet should be flat on the ground. There should be at least little space between the bar and your child's groin. A bike that passes all of these tests is the right measurement for your child.
Teach your child the importance of wearing a helmet at all times on a bike.