How To Measure A BMX Bike Frame

Author: Alex Bristol

Last Update:

Too big or too small?

When you’re trying to figure out which size BMX bike is right for you, you’ll probably find all sorts of measurements given. If you’re wondering how to measure a BMX bike, whether you’re buying your first BMX bike or an advanced cyclist and you want to check you’ve got the right size.

Whether you’re buying a BMX bike at home or getting fitted out at the shop, learning how to measure BMX bike frame is key to getting the fit you want and prefer. The best BMX bikes are only as good as their size, with the wrong size bike you’re going nowhere.

This is worth checking out.

I’ve got you covered. In this article, I will discuss everything I wish I had known when picking out my first BMX for myself. From learning how to measure BMX bike for adults to how to measure BMX bike frame and what all the little measurements mean and decoding the language so even beginners can have a go. If you are unsure of what size you need, have a look at what age is a 20inch BMX bike for?

Most things such as frame size and tube frame can be very different if you don’t know what you’re looking for. In a complete bike, you can have different tube lengths or chain stay length, which affects the bottom bracket and head tube angle as well. Depending on your riding style, you’ll want to check out a few things.

So, grab a measuring tape and let’s go straight in.

How are BMX bike frames measured?

Frame sizing can vary slightly. The general way of measuring a bike is by looking at the top tube length. You might have seen this quoted on bike retailers online or when shopping around, but all in all, it’s not a great method.

The top tube length is the center line of the top tube from the center line of the seat tube, to the center line of the head tube. The center of the seat and the center of the head is a good place to measure. That’s the main part of the bike frame, but if you’re comparing one bike to another it’s not ideal for working out sizing. You’ll also have to look at crank arms, crank size, wheel size as well as top tubes when looking at a new frame or a new bike, since it’s mostly personal preference, so nose manuals won’t help.

The top tube length may be the biggest measurement, but the angles of the other bike parts can make a huge difference to the overall fit of the bike. The most important size is how the bike feels to the rider.

A girl riding her bicycle in the middle of the desert

What to look for when measuring a BMX bike for yourself

Aside from the top tube length and other measurements, it’s important to know how the bike feels to you. I’m talking about all the contact points and riding positions.

1- Check where your feet are, where your hands reach, and where the bike meets the ground. This is going to give you an idea of your main points of contact and how comfortable it feels.

2- When looking at your feet position, it depends on many aspects of the anatomy of the bike. From the length of the crank to the pedals themselves. The bottom bracket height is something to consider too, since most of the weight is on there, it will decide how high off the ground you are.

4- Relativity. The key here is to look at how your feet, hands, and where the bike meets the ground are positioned relative to each other. This will give you a complete picture.

5- Decide if you want speed or stability. A bike with a longer wheelbase that is not very tall will be great for maintaining high speeds but will take longer to turn or maneuver. Bikes that have a shorter wheelbase and are taller can turn quickly and nimbly, but at higher speeds don’t provide a lot of stability.

Remember, the top tube length isn’t the most important measurement. Wheelbase can be similar but a small difference in size can completely change the feel of the bike. There’s also the degree of the seat tube. Having a degree steeper on the seat tube can make the frames front end longer, and half a degree steeper head angle can make the frame front shorter, so bear this in mind when comparing bikes with the same top tube length.

A man doing tricks on his bike in an empty place at sunset

Chainstay length

This is a measurement most people miss out on, but it gives a really good idea of the size of your BMX bike.

The chainstay length affects the total wheelbase, so a longer chainstay is going to give you a bike that is more stable but harder to maneuver. It can also affect how hard the front end of the bike is to get up and the stability.

Short chainstays can make it easier to pull up into a manual, but it’s not so easy to balance once you’re up there. Keep this in mind if you like to do street riding or stunts and tricks. Although chainstay length is a small part of the anatomy of the bike, it will affect the overall feel of the bike.

In general, I’ve seen mostly bikes that have longer front ends, with steeper head angles, the chainstays that are shorter. This helps the bike stay stable but also turn pretty quickly too.

Stem Length

The stem length is also a useful measurements for your BMX bike. Something to keep in mind however- a long stem will not make up for a short top tube. Just moving your bars forward will have the same effect as having a longer stem, but no one reasslly wants bars set steeper than your head angle right?

If you use a longer stem, you will have more room at the front but it won’t make the frame feel longer. The speed of the bike will be pretty much the same, as will the stability. The main difference will be in doing front wheel tricks. When you put your weight more forwards, it will make a difference to the feel of the bike when doing front wheel tricks.

How to measure BMX bike frame size for BMX bikes

If you have a BMX or want to buy a BMX but are not sure which measurements you need to determine if it’s the right size for you, then you’ve come to the right place. BMX bikes have similar anatomy to others, but BMX bikes can have varying measurements between each other. Jot down a pencil mark to help you.

1-Find midpoint between Top Tube and Seat Tube

I know, after all that I went on that the top tube length isn’t that important, it is slightly. Find the intersection of the top tube and the seat tube, this point is the first step to your measurement. The seat tube is the defining point for getting the top tub length, so start at the seat tube.

2-Find the midpoint of the top tube and head tube

Now from the top tube you’ll need to go towards the head tube and find the intersection of both, this will be your second point of measurement. The head tube marks the second point of measurement you need on all BMX bikes.

3-Measure both points

Now you have both points, you’ll need to measure the distance. This measurement will be the length of the top tube of the bike (TT). You can take this measurement in centimeters (cm) or inches (to the nearest half inch)

This measurement will give you an idea when looking at a bike chart what age and height are recommended for your BMX bike’s top tube length.

Final Thoughts

Next time you’re thinking about treating yourself to a new BMX, you’ll know not to just check the top tube length for frame size. Bikes with the same top tube length can have a very different feel from one to the other, so a good indication is what do you think about your current bike frame size?

Depending on the kind of cycling you do on your BMX, you can decide that you want a longer top tube on your frame size, or increase the speed or stability by playing around with the length of the chainstay or bottom bracket.

No matter what everyone else thinks is the right size for you, there’s nothing better than getting on a bike yourself and deciding how you want it. You’ll know what feels right, and once you have a bike that works you’ll know which measurements to go for.

Remember, even a quarter of an inch can make a whole difference in the feel of the bike.

Have you got a favorite BMX bike?

Photo of author


Col de la Madeleine marker with 2 bikes in front

How To Ride A Road Bike: Great Road Biking 101

Looking to get into road biking or just bought a road bike from your local bike shop spontaneously and completely forgotten how to ride a bike?
A man riding a bike down a dirt track

How To Clean A Mountain Bike

You’ve just taken your mountain bike out on a challenging trail and it’s caked in mud so now you’re wondering ...
A man carrying his hardtail mountain bike while looking at the view

What Is A Hardtail Mountain Bike?

Have you been looking for a new mountain bike and aren’t quite sure what a hardtail bike really is? Well, ...