Bike Sizing

How to measure your child's inseam

Stand your child against a wall in the shoes they will ride in. Put a hardcover book between their legs, spine up, with the pressure a saddle would apply. Mark the top of the spine on the wall. Measure floor to mark. That number is the inseam, and it decides the bike.

By Mike M.Published September 23, 2026Last updated September 23, 2026How we choose

The method

This is the same method woom publishes for its own bikes, and it is the one the rest of this site assumes you used.

Step 01

Shoes on, back to the wall

Put your child in the shoes they will actually ride in — a sneaker sole is worth half an inch and half an inch decides sizes at this scale. Heels against the wall, standing straight, feet a few inches apart.

Step 02

Book between the legs, spine up

Take a hardcover book and slide it up between their legs with the spine facing up, flat against the wall. Apply the same gentle upward pressure a saddle applies when they sit on it. Not hard enough to lift them.

Step 03

Mark the top of the spine

Hold the book level and mark the wall at the top edge of the spine in pencil. Have your child step away before you measure — they will fidget, and the mark will not.

Step 04

Measure floor to mark

Measure straight down from the mark to the floor with a tape. That is the inseam. Write it down in inches and in centimeters, because manufacturers publish in both and you will be converting all evening otherwise.

What to do with the number

Compare it to the minimum saddle height of the bike you are considering — not to the wheel size, and not to the age on the box. If the minimum saddle height is at or below the inseam, your child can sit and put their feet down. If it is above, the bike is too big, whatever the age band says.

A worked example. A five-year-old measures a 17 inch inseam. Prevelo publishes a 16.9 inch minimum saddle height for the 16-inch Alpha Two — that fits, with nothing to spare. woom publishes 19 inches for the 16-inch GO 3 — the same child would be on tiptoes at a stop. Same wheel size, same quoted age range, two different answers.

The full mapping lives on the kids bike size chart, and if you want to see how badly age tracks any of this, the wheel size by age page lays the brand disagreements side by side.

Two common mistakes

Measuring barefoot

Barefoot costs you around half an inch. At the 14-to-16-inch transition, half an inch is a size. Measure in shoes.

Measuring the pants inseam

Pants inseams are cut for hems, not crotches, and they run short by an inch or more. Use the wall.

How often to re-measure

Twice a year is plenty — and always before you buy anything. Kids add height in bursts, and a measurement from last winter is fiction by spring.

Questions parents actually ask

01How do you measure a child's inseam for a bike?

Stand them against a wall in riding shoes, raise a hardcover book spine-up between their legs with the pressure a saddle would apply, mark the wall at the top of the spine, and measure from the floor to the mark.

02Should I measure with shoes on or off?

Shoes on. A sneaker sole adds roughly half an inch, and half an inch is enough to change which bike fits.

03Is a pants inseam the same measurement?

No. Pants inseams are cut short for hemming and typically under-read by an inch or more. Measure against a wall.

04What if my child is between two sizes?

Take the smaller one. A slightly small bike is rideable and confidence-building; a slightly big one gets avoided, and an avoided bike costs a whole season.