The spine · applies at every stage
Bike sizing
Age does not decide bike size — inseam does. The full kids bike size chart, how to measure an inseam in two minutes, and what to do between sizes.
11 pages in this section
Everything else on this site depends on one measurement: your child's inseam. Not their age, not their height, and certainly not the age band printed on the box.
Here is the evidence for that, from the manufacturers themselves. For a five-year-old, woom recommends a bike needing a 19.1 inch minimum inseam. Prevelo recommends one that fits from 15.5 inches. Guardian publishes a 16.25 inch minimum leg length. Three brands, the same child, nearly four inches of disagreement — and all three are right, because they are describing different bikes for different legs.
| Wheel | Inseam | Height | Age brands quote | Stage |
|---|---|---|---|---|
| 12" | 10–18 in | 31–40 in | 1.5 – 3.5 | Balance bike |
| 14" | 14–19 in | 37–43 in | 3 – 5 | First pedal bike |
| 16" | 15.5–25 in | 40–47 in | 4 – 6 | First pedal bike |
| 20" | 18.3–28 in | 43–51 in | 5 – 9 | First gears |
| 24" | 26–32 in | 49–57 in | 7 – 11 | Big kid |
| 26" | 29.5–37.4 in | 55–65 in | 10 – 14 | Adult-adjacent |
Built from the inseam ranges woom, Prevelo, Guardian and Strider publish for their own bikes (read 2026-09-23). Age is the brands' figure, shown so you can see how loosely it maps.
How to measure
Shoes on, back to a wall, a hardcover book raised spine-up between the legs with the pressure a saddle would apply. Mark the wall at the top of the spine, measure from the floor. Two minutes, and it is the same method woom publishes for its own bikes. Step-by-step: how to measure your child's inseam.
What to do with the number
Compare it to the minimum saddle height of the specific bike you are considering. If the minimum is at or below the inseam, your child can sit and put their feet down. If it is above, the bike is too big — regardless of wheel size, brand or age band.
When a child lands between two sizes, go down. A slightly small bike is rideable and confidence-building. A slightly big one gets avoided for a season, and the bike that does not get ridden costs exactly as much as the one that does.
The two saddle heights
A learner needs the saddle low enough to sit with both feet flat. A confident rider wants it high enough that the knee is just short of straight at the bottom of the pedal stroke. Both are correct in their moment, and moving between them takes weeks, not minutes — kids bike seat height covers the transition.
Re-measure twice a year
Children add two to three inches of height a year at these ages, in bursts. A measurement from last winter is fiction by spring, and a bike set up in April is usually too low by August. Check the saddle at the start of each season, and never raise the post past its minimum-insertion mark.
If you are buying as a gift and cannot measure the child, there are three usable proxies and one safety net — how to buy a bike as a surprise gift.
Everything in bike sizing
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