Mountain Bike Sizing Guide

Get the most out of your mountain bike with our size guide!

Mountain Bike Fit

One of the most important aspects of buying a mountain bike is finding the one that fits you correctly. A bike that fits well and is right for your height, flexibility and riding style is a bike you’ll love riding, and a properly fitting bike can improve your handling and confidence on the trail to help you tackle more technical and challenging rides. If you want more information about what mountain bike is the right fit for you, view our Mountain Bike Buying Guide.

Mountain Bike Sizing

Many mountain bikes come in standard sizes (S, M, L) and are generally similar across brands. However, some brands have specific sizing methods. You can refer to the size chart below to get a general idea of what size mountain bike you should get, or you can also follow the links posted below to brand specific sizing guides.

Mountain Bike General Size Guide

Specialized Bikes Traditional Sizing Tool

Specialized Bikes S-Sizing Tool

Trek Bikes Sizing Tool

Brand Specific Size Charts

Get the best fit by going to a bike store: The best way to find out what mountain bike suits you best is to head to a specialty bike retailer to identify some suitable models and try out a few bikes. Ski Hut also offers personal bike fitting appointments for those who want to customize their fit to their specific needs.

How Are Mountain Bikes Sized?

Most mountain bike manufacturers use classic small, medium, and large sizing. For the most part, bike frame size is the only thing that changes when you go between sizes - things like wheel size, suspension, and relative geometry remain the same. However, some manufacturers will change things like wheel size and suspension travel across the size run of the same model.

Seat tube length is the traditional method for bike sizing, but with all the interesting shapes that mountain bikes come in these days, it has become less relevant to finding what size mountain bike you should get. Most mountain bikes focus on an extremely low standover height, so the traditional method of standing over the bike flat-footed doesn’t really work either. This is why mountain bike manufacturers use small, medium, and large sizing and give their own size recommendations based roughly on your height.

Sizing Up or Sizing Down Your Mountain Bike

So you’ve matched your height to a manufacturer’s mountain bike size chart, and you’re right on the edge - what do you do now? Sometimes you’re in between mountain bike sizes. Here’s a few tips on when to size up or size down your mountain bike when you’re in between sizes.

What You Get for Sizing Up

When it comes to mountain bike sizing, reach tends to be what changes more from small to large than stack. Almost all mountain bikes are trying to get as low as possible to buy the rider range of motion in the cockpit of the bike so you’ll see a lot of bikes with an increase in reach. There’s also a minor corresponding increase in wheelbase as the whole bike itself gets a little bit longer, so you get the benefits (and pitfalls) of a longer mountain bike. Also, if you happen to have a proportionally longer torso, you’ll want to size up in order to get a neutral fit, since reach is primarily what’s affected by the size. Flexibility and range of motion in your hips is also something to consider. If bending over and touching your toes sounds like a tall order, a longer bike is going to be a bit more taxing to maintain a low and aggressive riding position.

To sum up: If you’re flexible, longer in the torso and like to monster-truck over your terrain rather than whip or flick your way around it, size up your mountain bike.

What You Get for Sizing Down

Conversely, a smaller sized mountain bike will have a shorter reach and a slightly shorter wheelbase. Stand-over height will be a tad lower and you might have to extend your seat-post a bit in order to get to your pedaling position, but it’s a pretty minor consideration when it comes to being between sizes. If you’ve got proportionally longer legs than average, lack a bit of flexibility or generally like to be more upright , you may be a bit more comfortable on a shorter size. Additionally, a shorter wheelbase will ride more nimbly and while the longer cockpit provides a really aggressive riding position, low and aggressive riding positions are tiring to be in, so consider your fitness, endurance and ride length. Shorter bikes are easier to be on for long periods of time.

To sum up: If you like a more nimble and whippy ride, are a bit less flexible, are shorter in the torso or are looking for a more casual riding position, size down your mountain bike.

Mountain Bike Geometry Measurements

Mountain bike geometry is literally the shape of the bike. The overall shape and geometry of your mountain bike is made up by several different and important measurements. These measurements are important when discussing the fit, feel, and style of a mountain bike, and are often proportional to the style and terrain your bike was intended to perform on. The two main measurements that will affect the way that your bike will fit are reach and stack. Keep in mind that while these terms come from the road, triathlon & time trial world, those numbers won’t port over from your tri-bike to your mountain bike.

Mountain Bike Reach

The reach measurement on a mountain bike is the horizontal distance between the bottom bracket and the center of the head tube. This is arguably the most important figure for mountain bike fit because it affects the length of your bike’s cockpit when you’re standing on the pedals and how much range of motion in your hips you’ll use up in order to achieve a good, strong riding position. Too long of a reach and you’ll be stuck leaned over and stretched out, too short of a cockpit and your weight bias will shift too quickly and leave you in a position that’s overly upright.

Mountain Bike Stack

The next important mountain bike geometry measurement is stack, the vertical distance between the bottom bracket and the center of the head tube. This is primarily a gauge of seated pedaling position and relative handlebar height. This can be adjusted to a degree, with headset spacers & handlebar rise for increased stack height. For mountain bikes, where geometry is based on the aggressive riding (standing) position with the seat down, Reach trumps Stack as the primary fit dimension.

Mountain Bike Head Tube Angle

Head angle, or head tube angle, is the angle between the front fork of your mountain bike and the ground. Although there are other parameters that go into what makes the front wheel behave, (fork offset, trail, etc) this is the key metric that the industry uses to determine front-wheel characteristics. A “slack” mountain bike head angle is a lower number (eg: 65°) relative to a “steep” head angle (eg. 70°). A slacker head angle will, in general, be more stable at high speeds as well as feel more comfortable on steeper (downhill) terrain. Conversely, your bike will steer lazily and flop from side to side on uphill terrain - you will need to make more body position changes in order to tackle steep up hills, such as keeping your weight on the front tire so you don't wheelie yourself off the trail. Conversely, bikes with steep head angles feel like the front wheel is planted and steer nimbly on uphill terrain, while feeling twitchy and unstable on fast downhill terrain. If your inclinations (no pun intended) are towards pedaling uphill, you’ll pick a bike with a steeper head angle and vice-versa if you like to head down.

Mountain Bike Chainstay Length

Chainstay length preference is personal to each rider. This measurement effectively determines the distance between the rider’s center of mass and the rear axle. Bikes with short chainstays have the rear wheel closer to being “under” the rider. Short chainstays allow the rider to be more dynamic with their weight. This means easier manuals & wheelies and a general ease of getting the front wheel off the ground. This comes at the expense of overall wheelbase length, which contributes to stability at speed and over rough terrain. (Consider that unintentional wheelies while going uphill aren’t generally a desired trait.) If you want your bike to ride nimbly and like wheelies & manuals, short chainstays are your friend. If you prefer stability, go for a bike with longer chainstays.

Mountain Bike Bottom Bracket Height

The bottom Bracket height on your mountain bike is the distance between the ground and the center of your cranks. Some manufacturers refer to “Bottom Bracket Drop,” which is the vertical distance between the wheel axles and the bottom bracket. They effectively measure the same trait of the mountain bike, which is how low your center of mass is. Just like your lowered sports car, bikes with low bottom brackets corner like they’re on rails and transition from turn to turn effortlessly. The tradeoff here is that the earth isn’t flat, and trails aren’t typically paved smooth; the lower your bottom bracket, the more likely you’ll clip rocks, roots or logs you’re trying to hop over. If you’re a skilled and savvy rider, you can make sure your pedals aren’t down when passing over obstacles, but that means not pedaling over rough terrain and can be difficult to do when you’re brain’s starved for oxygen on a climb or you’re traveling at blinding speeds.

Mountain Bike Wheelbase

There are a lot of factors that contribute to the wheelbase length on a mountain bike including head tube angle, reach and chainstay length, which we’ve discussed previously. Total wheelbase is something of a culmination of all of these factors. The gist of wheelbase is that increasing the distance between your front and rear axles will increase stability, whereas shortening wheelbase will make a bike more maneuverable. Take note of where that increased wheelbase is coming from (Head tube angle? Chainstays?) and reference the above measurements to see how that’ll additionally affect the overall ride of your bike.

Conclusion

That’s a lot of information and though we've only covered the basics, this should give you enough background to pick a the right size mountain bike. That said, there is still no substitute for getting out there and test riding some bikes. When it comes to choosing a mountain bike, there’s always a little bit of an unexplainable, intangible connection to a particular ride. It just fits and feels right. So take this info with a grain of salt, throw your leg over a couple of bikes and go ride!