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About this tool
Pick a child's wheel size from their inseam, then set saddle height and standover clearance and see how long the bike will fit.
Children's bikes are sold by wheel diameter, and the measurement that picks the right one is the child's inseam — crotch to floor with shoes off — not their age. This selector maps that inseam onto the standard 12, 14, 16, 18, 20, 24 and 26 inch bands, then sets the saddle height using the classic 0.883 x inseam leg-extension rule and checks the top tube clears the inseam by at least 2.5 cm. It also estimates how many months the bike will fit before the next size up.
Open Kids Bicycle Size Selector on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Flags it when age and inseam disagree, and tells you which to follow.
Feet flat for beginners, balls of feet next, full extension when confident.
Uses roughly 2.75 cm of leg growth a year to estimate the months left in a size.
Measure their inseam and match it to the wheel size: roughly 35-42 cm for 12 inch, 40-48 cm for 14 inch, 45-55 cm for 16 inch, 55-63 cm for 20 inch and 62-72 cm for 24 inch. Age-based charts are a rough guide only, because two children of the same age can differ by a whole wheel size.
Stand them against a wall with shoes off and feet about 15 cm apart, hold a hardback book up between their legs with the spine level like a saddle, then measure from the top of the spine to the floor. That number in centimetres is the inseam and it is the single most useful measurement for bike sizing.
No, not for a beginner. A bike too big to stop safely on delays learning and causes falls, and the child rides it less. Buy to fit now, raise the saddle as they grow, and move up when the saddle runs out of post — most sizes give a year or two of use that way.
For a child still learning, set the saddle top level with their inseam so both feet sit flat on the ground when stopped. Once they can start and stop unaided, raise it until only the balls of the feet reach — about 0.95 x inseam — and for a confident rider go to roughly 0.883 x inseam measured from the bottom bracket, which gives an efficient, nearly straight leg at the bottom of the pedal stroke.