What Size Mountain Bike Do I Need – 2024 Guide

So, you have been through all of our mountain bike articles, and it has left you wondering, what size mountain bike do I need? Well, in this article, we are going to give you some ways to find out what size you need before you spend your hard-earned cash.

Unfortunately, gone are the days that you could pick the frame size that you know you are. Different bike manufacturers have different measurements, and points to measure from to determine the size. That means that if you have a specific brand of bike, and you now want to change to a new brand, you will have to find out what size mountain bike you need again.

Even if you search for the term “What Size Mountain Bike Do I Need?” you will probably come up against a lot of places that show you a table of the person’s height to frame size required. However, unless you buy their specific bike, it will probably not be as comfortable as you would like to expect.

Why Is It Difficult?

Honestly, as you can see from the above, there is not going to be one size that will fit you. You may get lucky and buy the same size bike from the same brand as you already own and get the perfect fit. But it is doubtful. The reason for this is because of the different shapes and styles of bikes and different shapes of people.

I remember the days that I used to go into cycling shops as a kid, and they would get you to sit on the bike with the seat all the way down, and if your feet were just flat on the floor, you were on the right size bike. Then they would adjust the seat height so that you were on your tiptoes, and off you go.

When you are talking about mountain biking, that is just not going to cut it. With the evolution of mountain-bike geometry for different variations of the sport, we now have t start looking elsewhere to the standard eighteen-inch frame.

So, What Size Mountain Bike Do I Need?

The question is less about what size mountain bike do I need, and more about what is the reach of the bike that you need. Take a look at the image below:

What Size Mountain Bike Do I Need?

In the picture, you can see that a vertical line takes the reach measurement from the bottom bracket axle and the center of the head tube. Of course, going and sitting on a bike is going to give you a good idea about if it is the right size for you or not, but unless you are going to take a plumb line or spirit level and measuring tape when you go to see it, you will struggle to know if it is the right size for you.

How Do You Measure Your Reach?

That is another hard question to answer. The reach measurement of a mountain bike indirectly explains the mountain bike size requirements, but not alone. The way that reach changes your size is by defining your cockpit when you are standing on the pedals. If the reach is too long, then you will always be leaning over and stretched out. If it is too short, then you will always be too upright with restricted hip movements.

Unfortunately, there is no way to adjust this, so you are best to try out a few bikes, find the reach that suits you and aim for that. Luckily, all of the top brands have included this measurement into their specs. Therefore, if your bike if the right reach for you, aim for one about the same.

What About The Stack Height?

The stack height is the next most crucial measurement in finding the right size mountain bike. That measurement is the distance vertically between the bottom bracket axle and the center of the head tube.

That distance is an excellent measurement to determine your seated peddling position. The stack height has got more adjustment than the reach, but still not a lot.

However, remember that in mountain biking, your riding stance is a lot more aggressive, and you will be standing a lot more of the time. Therefore, your reach is a lot more critical.

What Now?

Now that you have worked out your ideal reach and stack height, you can look on the manufacturer’s information sheets for the bike that you want and decide between the new standard of small, medium, large, etc. Unfortunately, that is not as simple as it gets. What happens if you buy one size, it is too small. So you return it to the very accommodating shop and get the next size up, and it is too low?

Well, now you have to decide if you want the larger or, the smaller bike. There are some things that you can do to increase the size of your mountain bike, but not a lot that you can do to decrease it. So you have to consider your riding style.

Sizing Up

Sizing up is an option that you have no matter what size bike you already have. So long as the reach length is ok. However, it is often the reach that determines the size in the new size standardization. Most mountain bikes these days try to keep you as low to the frame as possible so that you have the best possible range of motion. That may seem a little bit backward, but once you have ridden a mountain bike that sits you up, you will see what we mean.

So, if your torso is quite long relative to your body, you are going to want a more significant reach bike than someone who has a shorter torso. To put it simply, if you would rather power your way over obstacles instead of maneuvering around them, sizing up is going to be the best option for you.

Sizing Down

Sizing down is something that you want to think about a little bit more than sizing up unless you have a smaller torso relative to your body. If you size down, you will have some uncomfortable rides ahead of you if the reach is not long enough, and there will be nothing that you can do about it. If you have long legs compared to a short torso, then you will be able to raise the seat post to accommodate the peddling position.

A smaller frame will also have a shorter wheelbase, and therefore will allow you to nip around the obstacles that the counterpart will be rolling over. That could be an advantage if you are a little bit shorter and enjoy a more nimble ride.


When you are asking yourself what size mountain bike do I need? I hope that the information in this article has given you a little bit more of a chance at getting it right. However, with so many factors built into the size of an MTB, one piece of advice that I would give to you is this: Take your time and measure the bike that you have. If it is comfortable, then great. Go for something with similar specs. If it is not comfortable, try to work out what changes you would make.

We all know that there is another unfortunate fact in the internet age, you can usually buy things cheaper online than in a shop. However, if you can, perhaps go and try some bikes out, and measure the ones that feel the best. Of course, demoing bikes is going to be the absolute number one way of finding the bike that you want, but that is just not practical sometimes.

Feel free to take a look through our other mountain bike articles here:

Best Full Suspension Mountain Bike Under 1500

Best Mountain Bike Shorts

Best Mountain Bike Helmets