A child's ideal bike size is typically determined by the age of the rider. Sizes are calculated by measuring the diameter of the wheel, as opposed to seat height and frame size. Sizes are based on the average height of a child at a given age.
Boys (ages 3-9)
The recommended bike size for a 3-year-old boy is a 30 cm (12 inch) wheel diameter, while the suggested sizes for boys ages 4 to 6 is a 40 cm (16 inch) wheel diameter. For boys between the ages of 7 and 9, bikes with a 50 cm (20 inch) wheel diameter are recommended.
Boys (ages 10-14)
Boys between the ages of 10 and 13 should typically ride a bicycle with a 60 cm (24 inch) wheel diameter, while the recommended bike size for a 14-year-old is 65 cm (26 inches).
Girls (ages 3-9)
The recommended size for a 3-year-old girl is a bike with a 30 cm (12 inch) wheel diameter. The suggested size for girls aged 4 to 6 is 40 cm (16 inches), and for girls who are between the ages of 7 and 9, a 50 cm (20 inch) diameter is typically the best fit.
Girls (ages 10-14)
It is recommended that girls between the ages of 10 and 13 should use a bike with a wheel diameter of 60 cm (24 inches). The suggested size for a 14-year-old girl is 65 cm (26 inches).