How to Size a Mountain Bike: The Ultimate 9-Step Guide

How to size a mountain bike in 9 steps.

Is it vital to have the right size Mountain Bike? You could say it’s important to have the right size for anything, but with a Mountain Bike, it’s especially key. The wrong-sized bike can make trail riding a miserable experience. You could find yourself constantly fidgeting in your saddle, unable to get comfortable, or worse, having control issues on technical terrain. Conversely, the right-sized bike will have you feeling like you’re one with the machine and make even the most technical sections feel like a piece of cake.

Wondering how to size a mountain bike? It’s actually not as difficult as you might think. In this article, we’ll walk you through the process step-by-step so that by the end, you’ll know exactly what size bike is right for you.

You may jump to the relevant part by using one of the links below or read on for all the information you need to pick out to size the mountain bike.

#1 – How are mountain bikes sized
#2 – Sizing up or sizing down your mountain bike
#3 – Important geometry terms and what they mean
#4 – Getting the perfect mountain bike fit
#5 – Seat tube length and standover
#6 – Saddle height and crank length
#7 – Top tube length and reach
#8 – Seat angle and effective top tube length
#9 – Foot position and cleats

#1 – How are mountain bikes sized

The manufacturers of most mountain bikes use the standard Small, Medium, and Large sizing. With only a few exceptions, bike frame size is usually the only thing that differs between sizes; whereas things like wheel size, suspension stiffness levels, and relative geometry number often stay constant. Even though this might be the case with some manufacturers changing features like wheel size or travel across their models’ different sized versions. Even if their measurements appear to be almost the same on paper, ask an expert biker about bike fit and they’ll tell you that each bike feels and rides differently, even if its specs appear similar.

Getting a bike that’s the perfect fit can seriously improve comfort, control, and speed on the trail.

So, where can you figure out what size frame you’ll need? There is no one-size-fits-all answer since, within reasonable limits, you may modify your saddle, stem, and handlebar to make a somewhat incorrect fit seem acceptable.

There are general size guidelines for different types of bikes, but for the most accurate sizing, we recommend checking the manufacturer’s own size chart. These charts will list a suggested height range for each bike frame size they produce.

mountain bike size chart
Size Chart Example

#2 – Sizing Up Or Sizing Down Your Mountain Bike

If you’re on the upper end of a bike’s size range, it’s often suggested that you size up to the next frame. While this may give you more room to grow, it could also make for a less-than-ideal riding experience. A longer bike might feel more stable at speed and handle big hits better but could be more difficult to maneuver in tight, slow-speed situations.

Conversely, if you’re on the lower end of a bike’s size range, sizing down to the next smaller frame might make more sense. This will make the bike feel quicker and more nimble but could come at the expense of stability and comfort on rough terrain.

It’s ultimately up to you to decide whether the advantages of a larger or smaller bike outweigh the disadvantages. If you’re on the fence about which size to get, it’s always best to err on the side of caution and go with the larger size. You can always adjust stem length and saddle position to dial in the fit.

#3 – Important geometry terms and what they mean

There are a few key geometry terms that you need to be aware of when shopping for a mountain bike. These include:

mountain bike geometery

Mountain Bike Geometry Terms on How to Size a Mountain Bike.


The reach measurement on a mountain bike is the horizontal distance between the bottom bracket and the center of the head tube. It’s basically the length of the cockpit. A longer reach will stretch out a rider and provide more stability at speed, while a shorter reach will make a bike feel more agile in tight situations. This is the most essential figure for mountain bike fit since it influences how far your bike’s cockpit extends when you’re standing on the pedals and how much range of motion in your hips you’ll use up to get a good, strong riding posture.


The stack is the vertical distance between the bottom bracket and the head tube’s center. This is primarily a gauge of seated pedaling position and relative handlebar height. This may be adjusted to some extent with headset spacers and handlebar rise for higher stack height. Reach trumps Stack as the primary fit dimension for mountain bikes, since they are designed around an aggressive riding (standing) position with the seat down

Head Tube:

The head tube angle is the angle between the front fork and the ground. This, along with fork offset (or rake), determines how a mountain bike will handle. A steeper head angle (e.g. 68 degrees) produces a sharper, more nimble handling feel while a slacker head angle (e.g. 63 degrees) provides more stability at speed and on steep terrain.

Fork Offset or Rake:

Fork offset, or rake, is the distance between the front axle and the fork’s crown. This contributes to a bike’s handling feel along with head tube angle. Bikes with less offset (or rake) will have quicker handling due to less trail.


Trail is the distance between where the front wheel’s contact point is with the ground and where the steering axis intersects the ground. It is a key factor in how a bike handles, particularly at speed and when initiating turns. More trail provides more stability while less trail produces quicker handling. You can think of it this way: the longer the wheelbase, the more stable the bike will feel at speed; the shorter the wheelbase, the more nimble it will feel.


The chainstay is the part of the frame that runs from the bottom bracket to the rear dropout. Chainstay length 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 the 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.

Bottom Bracket Height:

The bottom bracket is the part of the frame that houses the crank bearings. The bottom bracket height is the distance from the ground to the center of the bottom bracket. This measurement has a direct effect on a bike’s stability and climbing/descending abilities. A lower bottom bracket will make a bike feel more stable at speed and while pedaling through rough terrain. This comes at the expense of ground clearance, which is important for technical riding and avoiding pedal strikes. Conversely, a higher bottom bracket will be easier to clear obstacles with but can make a bike feel less stable on steep & fast descents.


The wheelbase is the distance between the front and rear wheels. A longer wheelbase provides more stability while a shorter wheelbase improves maneuverability. This is primarily determined by the bike’s chainstay length and head tube angle.

#4 – Getting the perfect mountain bike fit

The key to having a bike that rides well and feels comfortable. The best way to find your perfect fit is to get a professional fitting from a qualified bike fitter. A bike fitter will take your measurements, analyze your riding style and preferences, and advise you on the appropriate size bike. We’ll do our best to break it down for you so you can perform it yourself because these services usually cost around $300 an hour. Here’s a fast step-by-step guide to help get you started.

The first step is to measure your inseam. To do this, simply stand against a wall with your shoes off and place a book between your legs as high up as possible. Mark the spot on the wall where the top of the book meets the wall, then measure the distance from the floor to that spot. This is your inseam measurement and will be the primary determinant of what size bike you need.

Next, you’ll need to know your height. With your shoes off, stand up straight with your back against a wall. Again, place a book between your legs as high up as possible and mark the spot on the wall where it meets. Measure the distance from the floor to that spot and you’ll have your height.

Now that you have your inseam and height measurements, it’s time to consult a sizing chart. Mountain Bike sizing charts are available from all major manufacturers and usually list both inseam and height measurements.

Once you have your inseam and height measurements, finding the right size bike is simply a matter of matching up your numbers to the chart. If you’re on the borderline between two sizes, it’s usually best to go with the larger size. This will give you more room on the bike.

Choosing the right bike is a process that takes time and effort. We’ve seen riders who’ve spent years on what they thought was their ideal bike, with the correct reach, saddle height, handlebar shape, perfectly aligned fork, and best mountain bike tires correctly inflated until they realized it wasn’t. They discover that a basic change, perhaps even a few basic changes, to that setup seems to make them ride better.

Sometimes, all it takes to change the way you feel about your bike is a small alteration like a different handlebar sweep, tire pressure, or suspension-fork sag. Such details can make big difference in how you ride. If you ride a bike that fits you properly, you will feel more in control. The bike should feel like an extension of your body, and you’ll be able to ride with more confidence and less fatigue.

Let’s take a look at the basic rules of bike fitting and the alternatives to consider, as well as the rationale behind them. Don’t think of a bike fit and set up as something rigidly written in stone. Experiment with our standards as a basis until you discover what works best for you.

#5 – Seat tube length and standover:

The most important factor in sizing any bike is the seat tube length, which is the distance from the center of the bottom bracket to the top of the seat tube. The second most important factor is standover height, or how much space you have between your crotch and the top tube when you’re standing over the bike with both feet on the ground.

Seat Tube & Standover Infographic

The seat tube should provide you with an acceptable standover gap – the distance between the top tube and your crotch – as well as usable standover clearance. To ensure this, stand back as far away from the handlebar as possible while standing over it, and make sure there’s at least an inch of space between the top tube and your crotch.

If you follow this advice, your bicycle frame will allow for a large range of adjustments at the seatpost. This is key to finding your ideal mountain bike seat height. This is especially the case for beginner and cross-country bikes.

The rules for choosing a bike frame vary depending on the shape of the frame. For example, if the top tube is low-slung, you’re looking to buy a downhill or enduro bike – which has an entirely different geometry from XC bikes.

Standover height and seat tube length are not the only factors you should consider when determining a bike’s fit. Additionally, it is important to sit on different sizes of bikes before making your purchase.

#6 – Saddle height and crank length

Saddle height is another key factor in bike fit and it’s relatively easy to get right. Simply lower or raise the saddle until your leg has a slight bend (30 degrees at the knee) when the pedal is at its lowest point.

Crank length is the distance from the center of the bottom bracket to the pedal axle. Most mountain bikes have 170mm or 175mm cranks, which do the job perfectly well for most riders. But if you have short legs, you may find the cranks are too long to turn without your knee bending excessively at the top of the stroke, resulting in the wrong muscles being used.

Shimano XT crank arm – 170mm

If you’re having trouble finding a bike with shorter cranks, you can always install a shorter set of cranks on your existing bike. This is a relatively easy and inexpensive process that can be done at home with the help of a bike mechanic.

Once you’ve sorted out your saddle height and crank length, it’s time to focus on getting the perfect mountain bike handlebar setup.

#7 – Top tube length and reach

The top tube length is also a key aspect to consider. The comfort and efficiency of your body on the bike are determined by top tube length, together with seat height, stem length, and handlebar position. To make matters worse, the reach number is more significant than the top tube itself, which often slopes. The top tube length has a variety of functions beyond just making you comfortable.

The top tube length is responsible for more than just your comfort. It also has a big impact on the bike’s handling characteristics. A longer top tube will make the bike feel more stable at speed, while a shorter top tube will make it easier to maneuver in tight situations.

Top tube length is a good way to gauge how a bike will feel when you’re sitting in the seat, but the reach figure is more important for when you’re standing up. This is especially true while descending, but transitioning from one surface to another can be smoother with this increased comfort. For example, an experienced cross-country rider may prefer a longer and stretched-out position, whereas a beginner might want to stay upright for extra comfort and less weight on their hands and wrists.

The reach of your bicycle is generally a compromise between comfort, control, and pedaling efficiency. Choose what works best for you, but avoid being too hunched or overly stretched out since this can cause back pain and other issues.

#8 – Seat angle and effective top tube length

The seat angle is the angle between the horizontal and the line running from the top of the saddle to the center of the bottom bracket. A steeper seat angle (76-78 degrees) will put you in a more aggressive, aerodynamic position on the bike. This is ideal for cross-country riding where you’re looking to go fast and save energy.

A less steep seat angle (74-76 degrees) will put you in a more upright position, which is better for climbing and technical riding where you need more control over the bike.

When you’re new to biking, it’s easy to make the mistake of sliding your saddle too far back. This can actually have negative consequences though, like making the steering feel less precise and not letting your suspension fork compress properly.

Sit further forward and you’ll get the most out of the fork, full use of the front tire tread, and a better handling bike.

This is all dependent on the reach being correct for you. As a general rule, if you drop a plumb line from the center of the saddle, it should cross the chainstays almost exactly halfway between the bottom bracket axle and rear wheel axle.

Mountain bike pedals come in two varieties: flat or cliplessWith flat or platform pedals, the ball of the foot is usually positioned comfortably above the pedal axle. Clipless pedals, on the other hand, maybe more difficult to set up correctly; as a result, it’s important to understand how to properly install clipless pedal cleats. Finding the ball of your foot and placing the cleat directly beneath is a good place to start when setting up clipless pedal cleats.

Five Ten Kestrel Pro Boa Shoes and Cleats

If your cleats are too far back, you’ll find it difficult to push down on the pedals with any power. If they’re too far forward, you’ll be putting too much strain on your calf muscles. However, if you intend to ride Enduro or Downhill, you will want to set the cleat back on the shoe as far as you are comfortable. This is so you can relax your calves and drop your heels on longer descents giving you better rear-end traction.

The ideal position is somewhere in between, with your feet parallel to the crank arms when viewed from the front. Once you’ve found the right cleat position, it’s important to also adjust the tension on your pedals so that the cleats don’t come out too easily.

After you have the final setup completed, experiment with it a bit. Once you get the hang of it, riding will feel natural and easy without any ankle-, knee-, or hip-twisting. This might take a few rides, but keep at it—it’s worth it. And once you find that sweet spot, make note of where your cleats are positioned by drawing a line around them so you’ll know where to replace them when the time comes.

We hope you found this 9-step guide on how to size a mountain bike helpful. Now get out there and start shredding those trails.

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