Kids' bike size guide by age and inside leg
Age is a rough guide and nothing more. Two seven-year-olds can differ by 15 cm in height, and a bike that fits one will be unrideable for the other. Use age to narrow the search, then use inside leg to make the decision.
That said, age is where everyone starts, so here is the table, followed by the measurement that actually decides.
The quick age table
| Age | Wheel size | Typical frame | Notes |
|---|---|---|---|
| 18 months - 2 years | 10-12 inch | Balance bike | No pedals; learn balance first |
| 2 - 4 years | 12 inch | Balance bike or first pedal | Some move straight to pedals |
| 3 - 5 years | 14 inch | First pedal bike | Often with stabilisers, briefly |
| 4 - 6 years | 16 inch | Pedal bike | The most common first "real" bike |
| 6 - 9 years | 20 inch | Kids' MTB or hybrid | Gears start to appear |
| 8 - 11 years | 24 inch | Kids' MTB or hybrid | Often the last kids' size |
| 10+ years | 26 inch or 700c | Small adult bike | Move to adult sizing |
Two rules that save money and tears:
Do not buy a bike to grow into. A bike that is too big is unsafe: the child cannot put a foot down, cannot reach the brakes properly, and will not enjoy riding it. Children grow out of bikes quickly enough that "room to grow" usually means two years of a bike they cannot control.
Do not skip the balance stage. A child who has spent a year on a balance bike typically learns to pedal in an afternoon, because they already understand steering and balance. A child who goes straight to stabilisers often takes months longer and learns to lean on the stabilisers instead of balancing.
The measurement that matters: inside leg
Stand the child against a wall, barefoot, and put a book firmly between their legs at crotch height, as if it were a saddle. Measure from the floor to the top of the book. That is their inside leg.
Then match it to the bike's minimum saddle height, which is the number that decides whether the child can touch the ground. A child should be able to put the balls of both feet down while seated, with the saddle at its lowest sensible position.
| Inside leg | Wheel size |
|---|---|
| 35-42 cm | 12 inch |
| 40-48 cm | 14 inch |
| 45-53 cm | 16 inch |
| 52-60 cm | 20 inch |
| 58-68 cm | 24 inch |
| 66 cm and up | 26 inch or small adult |
If the child is between two wheel sizes, take the smaller one. A 16-inch bike that fits properly is far better than a 20-inch bike they cannot control.
Brakes: the thing parents underestimate
Small hands cannot reach or squeeze adult brake levers. Look for:
- Short-reach brake levers, designed for smaller hands, so the child can brake with fingers on the lever rather than the whole hand.
- Two brakes. A single rear brake is not enough. Front and rear, both working.
- Coaster (back-pedal) brakes on very small bikes are common and fine for a first bike, but a child should learn hand brakes before moving to a bigger bike with gears.
Check the brakes yourself before the child rides: lift the wheel, spin it, pull the lever, and make sure the wheel stops. Do this every few weeks; cables stretch and pads wear faster on kids' bikes than on adult ones, because the bikes get treated badly.
Weight: the number nobody prints
A cheap kids' bike can weigh almost as much as an adult bike. A 16-inch bike at 12 kg is a miserable thing for a 20 kg child to pedal, and it is the main reason some children give up.
If you can, check the weight in the listing. As a rough target:
- 12-14 inch: under 8 kg
- 16 inch: under 9 kg
- 20 inch: under 11 kg
- 24 inch: under 13 kg
Steel frames and cheap suspension forks are the usual culprits. A rigid fork on a kids' bike is not a compromise; it is lighter, needs no maintenance, and a child does not have the weight to make suspension work properly anyway.
Gears: when to introduce them
Most 20-inch bikes and above have gears, and that is the right point to introduce them. Before that, a single speed is simpler, lighter and less to go wrong.
When gears appear, check:
- A grip shifter or a simple trigger shifter the child can operate with a small hand.
- A gear range that suits the terrain. A 6-speed 14-28 tooth cassette is plenty for a child on roads and paths.
- A chain guard or a chain that is properly tensioned, because a dropped chain is the most common mechanical failure on kids' bikes and it happens at the worst moment.
Safety checks before every ride
Five things, thirty seconds:
- Brakes — both work, lever does not reach the bar.
- Tyres — firm, not soft. Check with a thumb.
- Bolts — the stem, the seatpost clamp and the wheel nuts. Children's bikes work loose.
- Chain — on the ring, not slack enough to fall off.
- Helmet — level on the head, strap snug under the chin, two fingers of gap above the eyebrows.
Replace a helmet after any impact, even if it looks fine. The foam is designed to crush once.
When to move up a size
Move up when the child is at the top of the saddle height range, not when they turn a certain age. Signs it is time:
- The saddle is at its maximum safe height and the child's knee is still very bent.
- The child's knees come up near the handlebars when pedalling.
- The bike feels twitchy and unstable because the wheelbase is short relative to the child.
Signs it is not time, whatever the child says:
- They want a bigger bike because a friend has one.
- The current bike is fine but the colour is boring.
Buying second-hand
A used kids' bike is often the sensible buy, because they outgrow them so fast. Check the frame for cracks near the welds, especially where the top tube meets the seat tube, and check that the wheels are true by spinning them and watching the rim against the brake pads. A wobbly wheel is cheap to fix; a cracked frame is not.
Avoid any used bike with a bent fork or a frame that has been repainted over damage. And check the brakes work before money changes hands.