Grown-up bikes are sized by the frame. Children’s bikes are sized by the wheels, in inches. That is the first thing to know, because a “20 inch bike” means the wheels are 20 inches across, not anything else.

The second thing to know is that the age on the box is a guess. Two children the same age can be a head apart in height. Measure the child, not the birthday.

The short answer, if you already have the measurement: 12in wheels suit an inside leg of about 35 to 42cm, 14in 40 to 48cm, 16in 45 to 55cm, 20in 55 to 63cm, 24in 60 to 72cm, and 26in or a small adult frame from about 70cm up. Here is how to take the measurement, and what to do when a child sits between two sizes.

How do you measure a child for a bike?

Stand them against a wall in shoes, with a book pressed up between their legs as high as it goes comfortably. Measure from the top of the book to the floor. That is their inside leg, and it is the number that decides which bike fits. Height on its own is not enough, because two children the same height can have very different leg lengths.

Write the number down. You will want it when you are looking at bikes, and it is the thing a seller cannot argue with.

Which wheel size fits which child?

As a rough guide: 12 inch wheels suit an inside leg of about 35 to 42 cm; 14 inch, 40 to 48 cm; 16 inch, 45 to 55 cm; 20 inch, 55 to 63 cm; 24 inch, 60 to 72 cm; and 26 inch or a small adult frame from about 70 cm. The ranges overlap on purpose, because they are a starting point, not a rule.

Wheel size against inside leg. The ranges overlap on purpose: if your child sits in two of them, the smaller wheel is usually the better bet.

If a child’s inside leg sits in two ranges, the smaller wheel is usually the better bet. They will be more confident on it, and confidence is what gets a bike ridden.

The ages people quote for these sizes, 12 inch for a three-year-old, 20 inch for a seven-year-old and so on, are averages. They are fine for guessing which bikes to go and look at. They are not fine for deciding.

How should the bike fit when they sit on it?

Sitting on the saddle, the child should be able to put the balls of both feet on the ground. On a first bike, flat feet are better still. Their knees should not come up near the handlebars when they pedal, and they should be able to reach the brake levers without stretching their fingers.

That last one matters more than it sounds. A child who cannot squeeze the brakes properly will not use them, and will learn to stop with their feet instead. Most brake levers can be adjusted closer to the bar with a small screw. Ask.

Should you buy a bike they will grow into?

No. This is the mistake nearly everyone makes, and I understand why. Children grow fast and bikes cost money. But a bike that is too big is a bike the child is scared of, and a scared child does not ride. Then it sits in the shed, and by the time it fits they have lost interest.

Buy the bike that fits now. A second-hand bike costs a fraction of a new one, so moving up a size when they grow is not the expense it would be otherwise, and the one they have outgrown still has value to the next family.

A saddle that goes up and down gives you a year or so of growing room on most bikes. That is the right kind of growing room. A frame that is too big is not.

Do they need gears?

Not until they are on 20 inch wheels at the earliest, and often not then. Gears are one more thing to think about when a child is still learning to balance, pedal and brake at the same time. A single-speed bike is lighter, simpler, and has less to go wrong. Once they are riding confidently and starting to complain about hills, that is the time.

What about balance bikes and stabilisers?

A balance bike (no pedals, feet on the ground) teaches the hard part of cycling, which is balancing, without the child having to think about pedals at the same time. Most children who start on one go straight to a pedal bike without ever needing stabilisers. If you already have a small pedal bike, you can take the pedals off and use it the same way.

Stabilisers work, but they teach a child to lean the wrong way in corners, and that has to be unlearned later. If you can avoid them, do.

The short version

Measure the inside leg, not the age: 12in suits 35 to 42cm, 14in 40 to 48cm, 16in 45 to 55cm, 20in 55 to 63cm, 24in 60 to 72cm, 26in from 70cm
Both feet on the ground when sitting on the saddle
Fingers on the brake levers without stretching
Buy the size that fits now, not the one they will grow into
No gears until they are asking for them

If you are not sure, bring the child. It takes two minutes to see whether a bike fits, and no chart can do it as well as sitting them on it.

If this is their first bike, we have also written about getting one without spending a fortune, including the thing almost nobody checks: how much it weighs.

Once they have one that fits, the Tarka Trail is flat and has no cars on it, which is the easiest place in North Devon to let a child get confident.

What size bike does my child need?