How to Size a Kids' Bike

One season your little ripper fits their bike perfectly, and the next they're pedaling with their knees around their ears. Or maybe it's their first bike. How do you find a bike that fits, while building confidence? Here's the thing: buying used doesn't just save money — it actually makes sizing easier, because you're not stuck betting on a size they'll "grow into." Buy what fits now, get a full season of good riding out of it, then sell it and pass it on to the next kid when they outgrow it.

Measure Inseam First

The single most useful measurement isn't age — it's inseam. Two five-year-olds can need completely different bikes.

To measure it, have your kid stand against a wall in the shoes they'll actually wear riding, and slide a hardcover book up between their legs until it touches (gently) — then measure from the floor to the top of the book. That's your inseam number.

Match Inseam to Wheel Size

Kids' bikes are sized by wheel diameter, not age. Once you have the inseam, follow this guide to determine what size bike your child needs.

Treat age as a rough sanity check, not the deciding factor — inseam and height win every time.

Fine-Tune Seat Height

A few fit details worth knowing once you've got the right size bike:

• Balance bikes: seat height should sit about ½" to 1.5" below their inseam, so both feet stay flat and kids can slightly bend their knee while sitting to "run" the bike .
• Beginner on a pedal bike: Compare the child's inseam to the bike's minimum seat height (not standover height) as a first time pedaler needs to be able to touch the ground while their butt is on the seat.
• Training wheels (which we do not recommend): seat can go up to about 3" above inseam.
• Confident, experienced riders: 2" to 4" above inseam is fine, and lets them pedal more efficiently.

The Secondhand Advantage

Because you're not paying full retail, there's little reason to size up "for room to grow." A bike that's too big is genuinely harder for a kid to start, stop, and control. That's not a safety-neutral choice — it's a real tradeoff. Look instead for a bike whose seat can adjust up close to your child's ideal seat height, so you get the most riding life out of the size you've got — then sell it forward when they're ready for the next size.

The Bottom Line

Measure first, buy for today's kid, and when they outgrow it, list it and let it start someone else's next adventure.

Ready to find their size? Shop Gear · Got something they've outgrown? Sell Gear