I've been thinking about buying a BMX bike. Ideally, it'd be something that I can play around on, but also something that my nieces and nephew can ride when they visit.

I don't really have any experience with BMX bikes and, as I'm shopping around, I see quite a few that are labeled as "kids" or "adult."

It seems as if the top tube length is the main difference between the two, but I've seen some that are very close, like 1/2" difference.

Is there really a difference, or is it just marketing fluff? If there is a difference, how much of an effect will it have on the feel and handling of the bike? Are there other differences in kids vs. adult BMX bikes?

  • Bottom bracket height is the main thing, A bigger rider gets a higher bottom bracket for longer cranks. Design for intended target riders is another, like the addition of a coaster brake. Commented May 28 at 13:54
  • How big are your nieces and nephew? If there is a large difference in size in the riders then getting something that works for everyone is harder.
    – David D
    Commented May 28 at 16:44
  • @DavidD Yeah, that's what I'm concerned about. They're between 5 and 9. The five year olds don't really ride without training wheels though, so the idea is that it'd be a toy for the bigger kids when they're hear and me when they're not. :)
    – jimchristie
    Commented May 28 at 17:38
  • Not sure about size, but ‘kids’ bikes are almost always made of significantly less expensive components, because they’re expected to be used (and abused) heavily for 2-3 years and then swapped out when the child outgrows the bike, so price is a much bigger consideration there. At least in the US they also always have coaster brakes, even when this is a really bad idea for the intended cycling discipline. Commented May 29 at 11:20

2 Answers 2


Smaller BMX bikes (assuming you are looking at 20 inch wheels only) have smaller handlebars, shorter cranks, sometimes shorter frames (depending on what it is) and angles can vary. Kids bikes are usually cheaper and have cheaper parts.

According to The BMX DUDE

BMX Bike Size Chart By Height & Wheel Size

Wheel size Rider height Top tube length
12″ 2’4″ to 3′ 12″ – 13″
14″ 2’8″ to 3’6″ 14″ – 14.5″
16″ 3’2″ to 4′ 16″ – 16.5″
18″ 3’10” to 4’8″ 18″ – 18.5″
20″ 4’8″ to 5’2″ 18.5″ – 20″
20″ 5′ to 5’4″ 20″ – 20.25″
20″ 5’2″ to 5’6″ 20.25″ – 20.5″
20″ 5’4″ to 5’8″ 20.5″ – 20.75″
20″ 5’6″ to 6′ 20.75″ – 21″
20″ 5’8″ to 6’2″ 21″ – 21.25″
20″ 6’+ 21.25″+

In 20" wheels frames range from 18.5" to 21.25". As an example (not a recommendation), an online store that has filters for BMX bike top tube sizes between 18.52" and 21.95" when 20" wheel is selected.

Go ahead and get a bike that you like and does what you want.

Then, see if the kids can ride it. If they can't ride your bike, find something inexpensive for them to ride (maybe used). You don't really want them to ride your bike anyway :)


The crank length will likely be longer on an adult-size bmx.

  • 2
    How much longer? Would I be correct in assuming that a kid could still ride an adult-size BMX and vice versa?
    – jimchristie
    Commented May 28 at 15:33
  • 2
    Depends on how big the kid is. I have an old adult-size bmx and my six year old can definitely NOT ride it.
    – SimonL
    Commented May 28 at 16:01

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