What Size Bike Does My Child Need? The Complete Kids' Bike Sizing Guide

If there's one question we get asked more than any other, it's this one. And it's a good question to ask before you buy, because getting the size right is genuinely the difference between a child who falls in love with cycling and one whose bike ends up gathering dust in the shed.

Here's the thing that surprises a lot of parents: kids' bikes aren't sized like adult bikes. There's no small, medium or large. Instead they're measured by wheel size — and that, rather than your child's age, is what you're really matching to.

Why wheel size, not age?

Children of the same age can be very different heights, so age is only a rough starting point. What actually matters is your child's inside leg measurement — the distance from the floor to their crotch — because that determines whether they can sit on the saddle and still get their feet down. That sense of "I can stop myself if I need to" is everything when it comes to confidence.

So before you buy, stand your child against a wall with their shoes on, pop a book between their legs level with the top, and measure from the floor to the top of the book. That number tells you more than their birthday ever will.

The quick wheel-size guide

As a starting point, here's how wheel size lines up with age and height:

If your child sits between two sizes, our honest advice is to size down, not up. Which brings us to the most common mistake we see…

Don't buy "room to grow"

It's completely understandable — bikes aren't cheap, kids grow fast, and buying a size up feels like good value. But an oversized bike is harder to control, the brakes and saddle are out of easy reach, and a nervous child rides less. A bike that fits properly does the opposite: feet down, hands comfortably on the brakes, confidence building with every wobble-free metre. You'll get far more use out of the right size now than a too-big bike they're scared of for a year.

Lightweight matters as much as size

One more thing worth knowing while you're choosing. A bike that's too heavy is just as off-putting as one that's too big — and a lot of budget kids' bikes weigh almost as much as the child. Ranges like Squish and Hoy are built specifically around light frames and child-sized components, which makes a real, visible difference to how quickly a child takes to riding.

Still not sure?

If all this feels like a lot, don't worry — you don't have to work it out alone. Our kids' bikes range is organised by wheel size so you can shop straight to the right fit, and our 60-second Bike Finder will match your child to the right bike from a few quick questions. Or just give us a ring — we do this every day and we're always happy to help you get it right first time.

Get the size right, keep it light, and the rest tends to take care of itself.