How To Choose The Right Mountain Bike Size (With Mountain Bike Size Chart)

Qualified bike fitter and mountain bike expert Robbie Ferri walks you through finding the right MTB frame size

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reviewed by Rory McAllister
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One of the biggest mistakes I see mountain bikers making time and again is having the wrong size bike.

The wrong mountain bike size for your body makes you uncomfortable, causes injuries, stops you from pedaling efficiently, and can greatly affect your performance.

As a qualified bike fitter who also happens to be an experienced mountain biker, I’m here to help you with the first and most important step in choosing your MTB: finding the right mountain bike size for you.

In this guide, we’ll specifically be discussing the frame size of mountain bikes. If you’re interested in learning about mountain bike wheel sizes, check out Mountain Bike Wheel Sizes Explained here!

Alongside our mountain bike size chart, in this article we’ll be covering:

My shoes and mountain bike pedals, showing my mountain bike frame size.
© Robbie Ferri/BikeTips

Why Is The Right Mountain Bike Size So Important?

1. Comfort

The first and most important reason to ensure you use the right mountain bike size is comfort.

Although bikes are adjustable, they have a limited range in their size, and if you are not on the right sized frame, it can cause a very uncomfortable ride, ruining the experience and stopping your adventures or training. 

2. Performance

Next, we have performance.

If you ride the wrong-sized MTB frame, it’s going to be tough to perform. When it comes to mountain biking, control is key, and if you can’t move around the bike properly because it is the wrong size, it will knock those trail times right up.

3. Injury Prevention

Finally, we have injury prevention.

If you’re on the wrong size bike, you won’t be able to adjust and fit it to yourself properly. Not only will this cause discomfort, but it can easily end up causing injury and stopping you from getting back on the bike as much as you might like. 

The 3 Most Important Mountain Bike Frame Size Measurements

Instead of just telling you about the different mountain bike sizes and giving you the mountain bike size chart, it’s important to understand the key measurements that make up those sizes and how they affect the bike.

The “sizes” used to describe a mountain bike frame size (e.g. M, L, XL) are subjective, and can vary significantly between manufacturers.

Therefore, it’s a good idea to understand these measurements, so you can more accurately compare mountain bike sizes and how they’ll relate to your body.

These aren’t the only measurements (others such as reach and standover height vs rider inseam also matter), but these three are the most important; if they’re right, the other measurements likely will be too.

1. Seat Tube Length

Diagram demonstrating seat tube length on a mountain bike.
Diagram demonstrating seat tube length on a mountain bike. © Robbie Ferri/BikeTips

Unlike road bikes, mountain bike sizes are typically based on the length of the seat tube rather than the top tube.

The seat tube length is one of the most important measures of frame size for any bike, but is especially relevant for mountain bikes.

Seat tube length has a minimal effect on handling in itself, but determines the maximum and minimum seat height (AKA saddle height). If the seat tube length doesn’t suit the rider, it’s likely the rest of the bike’s geometry will be thrown off too.

It’s measured from the top of the seat tube (where the seat post is inserted) to the middle of the bottom bracket.

The longer the seat tube, the taller the rider can fit. Typically, this can range anywhere from 12″ all the way up to 23″. You can then further adjust the fit using the seat post it houses.

2. Effective Top Tube Length

Diagram demonstrating effective top tube length on a mountain bike.
Diagram demonstrating effective top tube length on a mountain bike. © Robbie Ferri/BikeTips

Next, we have the “effective” top tube length. This measurement gives a sense of how spacious a bike will feel from the saddle.

It’s a much more useful metric than the actual top tube length, which fails to account for the angle of the top tube. These angles can be extreme in mountain bikes, depending on the style.

Effective top tube length is measured as a horizontal line between the top of the head tube to where it meets the seatpost.

3. Stack Height

Diagram demonstrating stack height on a mountain bike.
Diagram demonstrating stack height on a mountain bike. © Robbie Ferri/BikeTips

Finally, we have stack height.

The “stack” refers to the vertical distance between the top of the head tube and the middle of the bottom bracket. Stack height dictates the minimum possible handlebar height with no spacers being used.

A taller head tube is better for taller riders and offers a more elevated handlebar position. A smaller head tube works with shorter riders and can offer a more aggressive position.

Stack height is a more useful measurement than head tube length, which is sometimes used to size mountain bikes, as it takes into account the head angle (the angle of the steerer tube relative to the ground), which can vary widely for mountain bikes.

Diagram showing head angle on a mountain bike.
Diagram demonstrating head angle on a mountain bike. © Robbie Ferri/BikeTips

A steeper head angle (more upright) increases steering responsiveness at slow speeds, at the expense of some high-speed stability. A slacker head angle increases stability and extends the front-center section of the wheelbase, but reduces the front wheel’s sensitivity to steering input.

Recently, there has been a trend for mountain bikes to feature increasingly slack head angles, as they help give the rider confidence when descending at speed, though for tight, technical trails, a steeper head angle may be beneficial.

Our Mountain Bike Size Chart – And How To Use It Properly

Mountain Bike Size Chart

Here’s our mountain bike sizing chart. This will be an excellent starting point when it comes to finding the right mountain bike size for you, but there’s a little more to it than meets the eye, which we’ll discuss below.

Mountain Bike Size Chart

However, bear in mind that this is only an approximate mountain bike size chart, with plenty of overlaps.

Depending on the bike brand or style, the key measurements described above can vary widely.

For example, a mountain bike frame with an 18” seat tube from Trek could be a “Medium”, but an 18″ frame from Canyon might be described as a “Large”.

What Do The Manufacturers Recommend?

The next way you will be able to find the right size mountain bike for you is to use the information from the manufacturers.

Often they will have size guides on their website, suggesting typical rider heights for different frame sizes.

Though using the manufacturer’s suggested heights might seem like the obvious solution, these are still subjective suggestions and often don’t differ between mountain bike styles or models, meaning they can sometimes be a little misleading.

Using The Three Key MTB Size Measurements

Therefore, the most precise option is to use the key measurements described above to compare the specific size of the mountain bike in various aspects.

To recap, the three most important measurements for mountain bike frame size are:

  • Seat Tube Length
  • Effective Top Tube Length
  • Stack Height

If you’ve owned or test-ridden a bike that’s felt like a perfect fit, measure it in these three dimensions and then examine the specifications of the new MTB frame you’re looking at buying in different sizes (typically available on the manufacturer’s website) to find a close match.

Alternatively, you can use these measurements to get a better sense of the true size of a mountain bike frame, comparing between different manufacturers to assess the size more clearly than purely going on the subjective “Large”, “Medium”, or “Small” descriptions.

You can also use these measurements to assess differences in how the bike will feel for you to ride.

For example, a longer effective top tube length will provide a more spacious feel, but too long and you could start to feel stretched out and like you’re having to extend too far to reach the handlebars.

Using A Bike Fitter 

The next option you have is to book an appointment with a bike fitter. Bike fittings are already very commonplace in road cycling, but many mountain bikers haven’t fully caught on yet – though maybe they should!

A bike fitter is a professional who adjusts your bike to fit your body perfectly. Many cyclists don’t know that you can do a mountain bike fit without a bike, so the fitter can advise you on choosing the correct size.

Many fitters have cycling rigs you can sit on, and they adjust to help you cycle as efficiently as possible while also factoring in your riding style and goals. After finding your perfect fit, they can recommend the correct size for you and help recommend the right bike. 

Test Ride!

Finally, you should always test ride!

Find a bike shop with the MTB model you’re looking at and ask them if you can come and have a go. It might not be easy finding the exact bike, but it is going to give you a good feel for the sizing.

Or, if buying second-hand, make sure the seller is local so you can test the bike for yourself before committing. While getting a bike shipped from afar is tempting if it’s a great deal and the postage is affordable, it’s not worth it if the bike arrives and doesn’t fit you right!

Should You Size Up or Size Down for Mountain Bike Frames?

A mountain bike frame might only be offered in four or five sizes – which inevitably means for some of us, our perfect sizes are going to fall in the gaps between the options available.

I have struggled with this a lot in my time cycling, as I often sit between a medium and a large frameset with many different brands.

So, should you size up or size down for mountain bikes?

Well, in my opinion, it comes down to your goals as a cyclist to know the best option. 

For example, if I were looking to ride casually or for long distances, I would size up. The longer reach and higher stack will give a more relaxed riding position and be much nicer on the back.

A smaller bike would make the position more aggressive, and make long-distance riding more challenging. 

If I were looking to race, I would personally size down. The aggressive position created by the shorter reach and lower stack height will make you slightly more aerodynamic, and because the races are short, comfort isn’t always the top priority.

Ultimately though, it comes down to personal preference for all mountain bikers. The best solution in this situation is to test ride both sizes if possible!

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Robbie has traveled the globe as an endurance athlete and bikepacker, breaking world records and competing in international ultra-cycling events such as the BikingMan series and the Transcontinental Race. He's also worked as an ambassador for some of the industry's leading names, including Shimano and Ritchey. If Robbie's not on a bike, he's either fixing them or out walking with his dog!

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