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Most 10-year-olds ride a 24-inch wheel mountain bike, so age is only a guide. You need to check your child’s height: 24-inch bikes fit kids from about 4’1″ to 4’11” tall with a 23 to 28-inch inseam, and kids move up to a 26-inch bike at around 4’8″ to 4’9″.
Kids’ mountain bikes are sized by wheel diameter, and the brands don’t agree on the cutoffs.
In this post, I’ll give you an inseam rule for the heights where two wheel sizes both fit, and also answer the age questions parents ask.
However, if your child is 4’9″ or taller, switch to my mountain bike size calculator.
Kids mountain bike size chart by age and height
Trek and REI both publish a guide, and their height ranges differ by a few inches. Here are the two columns as a range you can land in and not as one hard line.
| Wheel size | Age | Trek height | Trek inseam | REI height | REI inseam |
|---|---|---|---|---|---|
| 16″ | 3 – 5 | 3’5″ – 4’0″ | 16 – 22″ | not listed | not listed |
| 20″ | 5 – 8 | 3’9″ – 4’6″ | 19 – 25″ | 4’0″ – 4’5″ | 22 – 25″ |
| 24″ | 7 – 11 | 4’1″ – 4’11” | 23 – 28″ | 4’5″ – 4’9″ | 24 – 28″ |
| 26″ | 10 and up | 4’8″ and up | 25″ and up | 4’9″ and up | 28″ and up |
Trek puts 24 inch bikes at ages 8 to 11, and REI puts them at 7 to 11.
A tall 9-year-old and a small 12-year-old can both ride the 24, which is why I recommend you check the inseam before the birthday.
The 16 inch row is Trek’s only. I included it so you can see where the 20 inch starts.
Between two wheel sizes: use the inseam
The two brands say 4’5″ and again near 4’8″ to 4’9″. At those heights I’d go by inseam, and these are the cutoffs I use:
| Your child’s inseam | Wheel size |
|---|---|
| under 24″ | 20″ |
| 24″ up to 28″ | 24″ |
| 28″ or more | 26″ |
Measure barefoot against a wall, with a book pressed up between the legs like a saddle. Take the number from the top of the book to the floor.
A child at the edge of a row should take the smaller wheel if they ride tight, steep trails and the bigger one if they ride open, fast ones.
The smaller bike is the easier one to throw around, and the bigger one rolls over roots more smoothly.
What makes it a kids mountain bike
Youth mountain bikes come with knobby tires and a suspension fork, which is what separates them from a standard kids’ bike with the same wheel size.
If your child will ride dirt trails and not just paths, buy that kind.
You can also resist buying a size up so they can grow into it. A frame that’s too big makes the bars hard to reach and the bike hard to turn.
Younger kids: 12, 14 and 16 inch wheels
The chart above starts at 16 inches, but many parents land here with a toddler or a four-year-old.
However, according to some shops, the usual age bands are 12-inch wheels for ages 2 to 4, 14-inch for 3 to 5, 16-inch for 4 to 6, 20-inch for 5 to 8 and 24-inch for 8 to 12.
REI adds that a child whose inseam fits a 16 inch bike may also fit a 12 or 14 inch one, so the inseam gets you to a short list and the child’s confidence picks from it.
The age bands also overlap. A tall 4-year-old can skip the 14-inch and go straight to a 16, and a small 6-year-old can stay on the 16 for another season.
What should a kid’s mountain bike have?
Size is half the purchase. The parts decide whether your child enjoys riding or gives up on it. Check these:
- Brake levers: small hands need levers that sit close to the grip. If your child can’t pull the lever with two fingers while still holding the bar, the bike is wrong, however well the frame fits. Most kids’ levers can be adjusted for reach.
- Cranks and bars sized for kids: adult-length cranks make the pedals feel far away, and wide bars make steering hard. Real kids’ bikes come with shorter cranks and narrower bars.
- Hand brakes: they are fine on kids’ bikes and are what they’ll use on every bike after this one.
- Gears: skip them below 20-inch wheels. One speed is easier for a young child to learn on.
- Weight: a heavy bike is hard to lift, hard to turn and tiring on climbs. Cycling Weekly weighed a 24 inch kids’ bike at 22.7 lb, and that is the sort of number to look for.
The 24 inch size is the crossroads where suspension becomes an option. Below that, most bikes are rigid, and the fork matters less than the fit.
A five-minute fit check
A child who can’t reach the brakes or the ground will lose confidence quickly. Therefore, you need to run these checks with your child on the bike, in their riding shoes:
- Feet: they should touch the ground with the balls of both feet while sitting on the saddle. Flat-footed is even better on a 16-inch bike.
- Standover: they can stand over the top tube with a couple of fingers of room.
- Brakes: they can pull both levers hard without letting go of the grips.
- Reach: their arms bend a little when they hold the bars. Locked elbows mean the bike is too big.
- Turning: they can steer a tight circle in a parking lot without leaning the whole bike over.
If one of these fails, try the next wheel size down before you buy new parts.
Can a child grow into a bigger bike?
A bike that is one size up makes a child reach for the bars and struggle to turn, as I said above, and that hurts the first rides the most.
So a better plan is to buy the right size and look for a bike with a seatpost and stem that adjust, so it lasts through a full growth spurt in its size range.
When the inseam crosses 28 inches, move to a 26 inch bike and take your next steps with this mountain bike size calculator.
Kids mountain bike sizing FAQ
What size mountain bike for a 10 year old?
A 24-inch wheel in most cases. It fits kids from about 4’1″ to 4’11” with a 23 to 28 inch inseam.
A very tall 10-year-old with a 28-inch inseam should try a 26.
What size mountain bike for an 8-year-old?
A 20 inch if they’re under about 4’5″, and a 24 inch if they’re taller.
Both brands put 8-year-olds at the end of the 20-inch range and the start of the 24-inch range.
What size mountain bike for a 12 year old?
A 26 inch if they’re 4’8″ or taller, and a 24 inch if they aren’t.
Trek lists the 26 inch for ages 10 to 12 and for 13 and up, so a 12 year old’s height is the better guide.
Is a 24 inch or 26 inch mountain bike better for an 11 year old?
Go by inseam. Under 28 inches, take the 24. At 28 inches or more, take the 26.
What size mountain bike for a 5 year old?
A 16 inch if they are under about 3’9″, and a 20 inch if they are taller.
A five-year-old on the line should try both and keep whichever lets them touch the ground and reach the brakes.
Does a kid’s mountain bike need gears?
Not under 20 inch wheels, single speed bikes are easier for a young child to learn on.
Is a 20 inch mountain bike too big for a 4 year old?
Yes, in most cases, most 4-year-olds fit a 14- or 16-inch bike.
Measure the inseam and check that they can reach the ground and the brakes.
