What size bike does a child need?

Measure your child's inside leg and match it to a wheel size: 44 to 52cm suits a 16 inch bike, 52 to 62cm a 20 inch, 60 to 72cm a 24 inch. Age charts are unreliable because healthy children of the same age differ by around 20cm in height, which spans two wheel sizes. Once you have the wheel size, the number that actually decides the fit is the bike's minimum saddle height, which should be no higher than the child's inside leg.

Kids bike size chart

Read this left to right. Inside leg is the measurement that matters, height is the fallback, and age is there only to orient you. The bands overlap because real children do.

Typical UK figures across the common kids brands, not a manufacturer spec. Always confirm against the specific bike.

Why age is the wrong thing to shop with

Nearly every kids bike chart is keyed on age, and age is the weakest signal available. On the UK growth charts, a healthy six-year-old sits anywhere between roughly 106cm and 125cm tall, and everything in that band is normal. That spread is wider than the gap between a 16 inch and a 20 inch bike.

So an age-keyed chart gives a confident answer that is wrong for the tallest and shortest children in every class. It is also the reason so many second-hand kids bikes are barely ridden. The bike was bought to match a birthday rather than a leg.

How to measure a child's inside leg

Inside leg and inseam are the same measurement. It takes a minute and a hardback book.

1. 1Stand them against a wall in socks, feet slightly apart.
2. 2Put a hardback book between their legs, spine upwards, pressed gently up to the crotch the way a saddle sits.
3. 3Mark the wall at the top of the book.
4. 4Measure floor to mark in centimetres, then do it once more to check.

The one number to ask a second-hand seller for

Wheel size is a label, not a measurement. Two 20 inch bikes can differ by 8cm in how low the saddle goes, which is the difference between a child riding confidently and a child who cannot touch the ground. Ask the seller for the minimum saddle height, measured from the floor to the top of the saddle with the seatpost pushed all the way down.

Ask for the maximum too. The gap between the two figures is how long the bike will last before it is outgrown, and it is the honest way to compare a cheap bike against a more expensive one.

Do not buy a size up for them to grow into

It is the most common mistake and it is an expensive one. A bike that is too big is harder to balance, harder to stop, and harder to pick up after a fall. A child who feels unsafe on a bike rides it less, learns slower, and asks to go out on it less often.

Buying the right size and selling it on when it is outgrown usually costs less overall, because kids bikes hold their value unusually well. Every parent needs the next size up, which keeps demand steady for the one you are finished with. When that day comes you can list the outgrown bike and put the money towards the replacement.

What to check on a used kids bike

Children's bikes are usually outgrown rather than worn out, so a used one is often in near-new condition. These are the checks that are specific to a child's bike rather than the generic used-bike list.

The last one takes seconds. Our free frame number check runs a bike against the UK stolen registers before you hand any money over.

Common questions

What size bike does a 6 year old need?

Most six-year-olds are on a 16 inch or 20 inch wheel, but age is a poor guide. Healthy six-year-olds vary by around 20cm in height, which spans two wheel sizes. Measure your child's inside leg instead: 44 to 52cm points to a 16 inch bike, 52 to 62cm to a 20 inch. Then check the specific bike's minimum saddle height is no higher than that figure.

How do I measure a child's inside leg for a bike?

Stand them against a wall in socks, feet slightly apart. Put a hardback book between their legs, spine up, pressed gently to the crotch like a saddle. Mark the wall at the top of the book, then measure from the floor to the mark in centimetres. Do it twice. That number, not their age, is what you shop with.

Should I buy a bigger bike for them to grow into?

No. A bike that is too big is harder to balance, harder to stop and slower to learn on, and a child who feels unsafe on it rides it less. Buy one that fits now and sell it on when it is outgrown. Kids bikes hold their value better than adult bikes precisely because everyone needs the next size up.

What is the difference between wheel size and frame size on a kids bike?

Children's bikes are sold by wheel diameter in inches, adult bikes by frame size. Wheel size is only a rough proxy for fit: two 20 inch bikes can differ by 8cm in minimum saddle height. Always confirm the saddle height rather than trusting the wheel figure alone.

When does a child move from a balance bike to a pedal bike?

When they can scoot confidently with both feet off the ground for several seconds, usually somewhere between three and four and a half. Readiness matters more than age. A child who has properly learned to balance often skips stabilisers altogether and rides a pedal bike within a day.

Is a second-hand kids bike safe?

Usually, and often better value than a new supermarket bike at the same price. Kids bikes are typically outgrown rather than worn out, so a used specialist bike can be in near-new condition. Check the brakes reach and work, that the tyres are not perished, that nothing is bent, and run the frame number through the stolen registers before you pay.

What size bike should I buy for a child as a present?

If you cannot measure them, use their current shoe-off standing height rather than their age, and keep the receipt or agree with the seller that you can swap sizes. Failing that, ask the parent for the inseam. It is a less romantic question than a surprise, but a bike that does not fit gets ridden once.

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