Kids Bike Size Calculator

How to use the calculator

1. Measure their height barefoot, standing against a wall. It’s the number that matters most.
2. If you can, measure their inseam: back against the wall, with a book pressed up between their legs like a saddle, measure from the floor to the spine of the book. That tells us whether they’ll reach the ground.
3. Enter their age. If you don’t know their height, we use the median height for that age. It’s only an estimate: in the CDC growth charts, a 6-year-old can be anywhere from about 42 to 49 in (107-124 cm) tall (5th to 95th percentile).
4. Tell us if they already ride. If not, we’ll tell you whether they should start on a balance bike.

The result gives you the wheel size, a second size that could also work, how much a good bike in that size should weigh and how to check the fit in the store. If you enter an age, you’ll find a link below to the full guide for that age.

How we calculate it

Kids bikes are sold by wheel size in inches, but what matters is whether the child can reach the ground and the handlebar. We compared the size charts from Woom, Decathlon (Btwin), Islabikes, Trek and Frog and built a consensus chart by height:

Then we apply three rules:

1. Where sizes overlap (for example, 46 in, or 117 cm, works on both a 16” and a 20”), we pick the bigger one only if the child is already in the lower-middle part of its range. Otherwise, the smaller one: a bike that’s too big is scary and slows learning.
2. If their inseam is short for their height (below the minimum for that size), we drop one size.
3. If they don’t ride yet and are under 39 in (100 cm) (or younger than 3½), we recommend starting on a balance bike.

Without a height, we estimate it from the median height for the age in the World Health Organization (WHO) growth charts. For ages 2 to 12 those medians are within about 0.7 in (1.7 cm) of the CDC growth charts used by US pediatricians: in the CDC charts, the median is about 34 in (86 cm) at age 2, 43 in (109 cm) at 5, 50 in (128 cm) at 8 and 59 in (150 cm) at 12, averaging boys and girls.

How much brands differ

• Balance bike: Woom lists 32 to 39 in (82-100 cm); Decathlon, 33 to 41 in (85-105 cm); Trek, 34 to 40 in (86-102 cm).
• 16”: Woom and Decathlon, 41 to 47 in (105-120 cm); Trek, 39 to 46 in (99-117 cm); Islabikes allows it from 39 in (98 cm).
• 20”: Woom, 45 to 51 in (115-130 cm); Decathlon, 47 to 53 in (120-135 cm); Trek, 45 to 52 in (114-132 cm).
• 24”: Woom, 49 to 57 in (125-145 cm); Decathlon and Trek, from 51-53 to 59 in (130-135 to 150 cm); Islabikes, from 50 in (127 cm).

The difference comes from the minimum saddle height and the geometry. So if your child is on the edge, compare their inseam with the minimum saddle height of that specific bike, which is usually on the spec sheet: for example, 15.5 in (39.4 cm) on the Woom 14” and 19.0 in (48.2 cm) on the 16”.

Balance bike or training wheels

A balance bike teaches the hard part (balance) and leaves pedaling, the easy part, for later. Training wheels teach pedaling but not balance: the day they come off, the child has to learn all over again.

The research points the same way: in a survey of 2,005 families in Portugal (Mercê and colleagues, 2022), children who started on a balance bike rode on their own at 4.2 years on average; with training wheels, at 6, and on a regular bike alone, at 7.3. In the Netherlands, with 173 children aged 4 to 6, those who used a balance bike started earlier, practiced for less time and rode independently sooner (Blommenstein and van der Kamp, 2022).

If your child is already too tall for a balance bike and can’t ride yet, there’s a trick: buy a pedal bike that fits, take the pedals off (and lower the saddle) for the first few weeks, and put them back on once they glide with their feet up.

How to check the fit

Balance bike: seated, with both feet flat on the ground and knees slightly bent. The saddle should be 0.5-1 in (1-3 cm) below their inseam.

First pedal bike (14” and 16”): seated, they touch the ground with the balls of both feet (at first, flatter feet are better). They reach the handlebar with arms slightly bent and can squeeze the brake levers with two fingers.

20” and 24”: with the pedal at the bottom, the leg is almost straight; seated, they can still touch the ground with their toes. The seatpost should never go above its minimum insertion mark.

Weight, brakes and gears

• Weight: look for a bike that’s no more than 30% of the child’s weight. A 4-year-old weighs about 35 lb (16 kg): their ideal bike is under 11 lb (4.8 kg). Among 14” bikes there are some at 11 lb (5 kg, Woom GO 2) and some over 15 lb (7 kg, Btwin 100).
• Brakes: a coaster brake (braking by pedaling backward) is intuitive for little kids, but it stops them from spinning the pedal back to get ready to start. Woom doesn’t fit them; Trek sees them as a first step until the child has enough hand strength. What matters most: small brake levers they can reach with two fingers.
• Gears: not needed until 20”. 20” bikes usually come with 1 or 7 gears, and 24” bikes with 6-8.

Common mistakes

• Buying by age without measuring. Two 6-year-olds may need a 16” and a 20”.
• Buying big “to grow into.” They can’t reach the ground, get scared and take longer to learn.
• Looking at price instead of weight. A heavy bike handles worse, is harder to ride uphill and tires them out.
• Leaving the saddle low once they pedal. Pedaling with their knees up by their chest is tiring: raise the saddle as they grow.
• Forgetting the helmet. The right size, buckled snugly, with no cap underneath.

There’s a guide for each age in bike size by age. For a teen who’s already past 5 ft 1 in to 5 ft 3 in (155-160 cm), use the adult bike size calculator.

Updated October 19, 2026 ·Report an error on this page