What Size Bike Should My Kid Ride?

The BMX bike industry has taken bike size and literally ridden away with it. For many new families the number of sizes to choose from can be overwhelming. When you were a kid you might have just been happy to have a bike to ride at all, and were even luckier if you had a choice between small, medium and large.

A Walmart bike was awesome and had an equally good price tag.

Enter in the BMX bike.

With the average BMX manufacturer selling between 8 to 12 bike sizes with some sticker shock price tags to boot, the concept of BMX bike sizing can be overwhelming.

So why should you care?

Why can’t you just buy the first used bike on the fence at the right price?

Well size matters. All jokes aside it is an important part of your rider’s comfort and success. Now I didn’t say price matters because that’s another (book size) debate, but size does.


 There is only about a 5 inch difference between what the average 5 year old might ride to what an average adult might ride. So 5 inches for 12+ years makes even a half inch seem monumental.

If A Bike Is To Big:

If a bike is too big there is added weight, extra g-force, and a lack of control. Getting out of the gate is hindered and let’s face it learning to balance on a bike meant for someone 3 years older than you is difficult.  As bikes get bigger so does the tire and tubing which compounds these problems.

If A Bike Is To Small:

 Riding too small of a bike creates difficulties as well. Feeling hunched over, a different center of gravity, the bike is too light, the tires too thin, may make you feel like you are unsafe and can throw your weight forward. Trying to balance on the gate and put intense pressure on your start is not fun when you feel like you are riding a mini bike.

It’s A Safety Issue:

Is safety a concern? Sure it is. We have all seen the wobbling and the head over handle bars. Frames and bike parts also have weight limits. Usually around 100 pounds.  You wouldn’t put your 115 pound kid on a kayak rated for 90 pounds would you? Or in a helmet that could fly off their head in a crash from being too big?

The control and safety from a proper size bike and parts is essential. If your budget is a concern you should know that the BMX used bike market is huge for both buying and selling. If you choose to go the custom route get quality parts that can be switched out as you get bigger frames. Usually frames junior and below can use the same 1 inch based parts and Expert and higher use 1 1/8 inch parts. If you shop right, your 4 year old can use a 20 inch wheel set through 4 different frame sizes, Nano to Junior, with just a few adjustments.


Rider sizing is the most asked, and most debated question in BMX. I wish there was a simple answer but there isn’t. However there are size charts and general rules of thumb that help. I could even take a guess based on your rider’s age, but true sizing is an art form that should be done in person and should hopefully take into account your rider’s unique riding style and physical characteristics. 

SIDENOTE: Need help with your sizing? Find a local track coach or someone who has a length of experience with riders around the age you need. Or call your preferred bike manufacturer. They should be more than willing to help you with questions and get you set-up on the right size for your rider’s  safety and success. 

So what you need to know is the basic lingo and options for sizing?


First there is frame size. Companies today make bikes in sizes from Picos and Nanos, for kids coming off of balance bike racing, to XXXL super tall adults. In general the bikes have a size name that is used by just about everyone in BMX and from smallest to largest here they are:

Pico, Nano, Micro Mini, Mini, Junior, Expert, Pro, Pro XL and Pro XXL

Now some manufacturers throw in some special sizes for those awkward growth years such as an Expert XL. Or go even longer in length. But when you ask your local bike expert what frame should my kid ride, you will most likely get one of these bike size names.

So what’s in a name? The frame size name is supposed to correlate to a Top Tube size. What is a top tube? It is the tube on the bike that runs directly between your rider’s legs when they are standing with their feet on the ground. The length of the top tube is measured (from the center of the head tube to the center of the seat tube) in inches from a Nano at 15 inches, a Micro Mini at about 16.5 inches all the way to a Pro XL at about 21.5 inches. Unfortunately these lengths can differ slightly between manufacturers which makes rider sizing even more difficult and makes top tube length even more important.

Knowing the size name will help you search for your perfect bike and knowing the top tube length will help you make sure that it’s the perfect fit. A general rule of thumb for kids ages and bike size can help you get started in the general area but is only a starting point and not an absolute.

  • Pico and Nano (2-4 years old),
  • Micro Mini (4-5 years old),  
  • Mini (5-6 years old),  
  • Junior (7-9 years old),  
  • Expert (9-12 years old),  
  • Pro (12-15 years old),  
  • Pro XL (14 and over).

I have a feeling I have left myself open to some comments on these age ranges but as I said it is only a general starting point.


Several companies, including Dan’s Comp, have general rider sizing charts that are a great place to start. They use a rider’s height and inseam as a guide to suggest frame size and crank length.

So why are they not exactly what you use?

Because they are a bit too general and are unable to take into act your rider’s arm length (reach) and whether they like a bigger feel or the control of a smaller frame.

Why do you know all of this information but still feel lost?

Because rider sizing is difficult. Kids grow. People have unique physical characteristics and it’s just plain hard to decide. If you are new take this information at face value and get your kid on a bike ASAP. They need to ride a bike before all of the “track experts” can start giving you advice. You need to watch their gate, their first straight and their rhythm section before the debate can begin. But my best piece of advice; find one person you respect and has been in BMX specifically for a good period of time. You may find the local new dad who has been there 3 months trying to give you advice on bike sizing and just because they “act like they know” does NOT mean they do. I have even had experience where a new dad was trying to give me advice and telling me to put an 8 year old on a Pro (wrong). Try not to get 10 people involved because you may get 10 different and confusing opinions.

Also know your budget ahead of time. It is very easy to be talked into a $2000 custom build instead of a $600 boxed complete. I will also say size is very important. Please do not get a bike “they will grow into”.  From a Micro (16.5 TT) to a Pro XL (21.5 TT) there is only a 5 inch difference between what a 5 year old might ride to what a full grown adult might ride. That means every inch counts and every half inch counts too.