What size mountain bike do I need? Plus tips on bike fit

"What size mountain bike do I need?" It's a question asked frequently – for good reason – because choosing the correct-size bike is one of the most important decisions you'll make.

We recommend you don't buy a bike until you've understood how to get one that's the right size for your height and body shape. Our ultimate guide to bike geometry and handling will help.

The best mountain bike for you, that fits correctly and is set up properly, will be a joy to ride, making it easier to tackle trails faster with more control. But a bike that's too small can be twitchy, nervous and uncomfortable on longer rides, technical descents or when you're just pootling along the flat.

Read on for some advice on what mountain bike frame size you should be considering, especially if you're in any doubt about it.

We'll also help you understand the changes you can make to your bike's parts to help it perform better for your measurements, personal requirements and preferred discipline. For example, the best mountain bike suspension forks can be adjusted to suit you.

Before we get started, if drop handlebars are your thing, then head to our guide to road bike sizing.

• What size mountain bike do I need?
• Important geometry terms and what they mean
• Getting the perfect mountain bike fit
• Problems caused by the wrong size mountain bike
• How to get the perfect mountain bike fit
• Components that affect comfort and control
• Tyre and suspension setup and pressures

What size mountain bike do I need?

Ask an experienced rider about bike fit and they’ll tell you that all bikes feel and ride differently, even if their numbers look almost the same on paper.

Manufacturers’ listed mountain bike frame sizes can be confusing. How to measure a bike frame is not set in stone. The traditional method is to list the seat tube length, but even that varies because some are measured to the top of the seat tube and some to the middle of where the top tube joins the seat tube.

Many manufacturers simply list their bikes as S, M and L, perhaps with XS or XL at either end.

And, more recently, bike manufacturers have begun listing their bikes' sizes based on reach figures rather than seat tube and top tube lengths.

This means they've been able to grow the bike's reach figure, wheelbase and top tube length while trying to keep seat tube lengths and stack heights shorter and lower.

Smaller seat tube lengths mean shorter people can fit on bikes with a longer reach figure because they can adjust the seat height lower, opening up the potential to ride a larger bike.

It's still important to consider seat tube and top tube length when buying a bike. The seat tube length will dictate the lowest saddle height that can be set and the top tube length will roughly dictate how stretched out a rider will feel.

So, where do you find out what size frame you need? Like so many other things on a mountain bike, there is no one perfect solution because, within sensible limits, you can adjust your saddle, stem and handlebar to help make a slightly imperfect fit feel fine.

We'd always recommend looking at manufacturers' own size charts, which will usually list a suggested height range for each bike frame size they produce, but here are some general guidelines:

Important geometry terms and what they mean

We all come in different shapes and sizes, and so do most mountain bikes, so we recommend using the information below to help you understand what size mountain bike frame you should be riding.

First, it's good to know the anatomy of a mountain bike, because we'll be referring to these terms.

• A. Seatstay
• B. Chainstay
• C. Seat tube
• D. Top tube
• E. Down tube
• F. Stem
• G. Head tube

When you're looking at buying your next bike, it's crucial to understand how the bike's geometry will affect how it rides and what each element of its geometry means. Understanding these terms will help you decide what size mountain bike you need.

When you're looking at mountain bike frame size charts, these terms will also help you understand how each measurement affects the size of the bike.

• A. Effective top tube length is the length of a virtual horizontal line drawn between the top of the bike's head tube and the centre of the seatpost at the same height.
• B. Stack height is the distance between the centre of the bottom bracket and the centre of the top of the head tube. This measurement dictates the minimum height of the bars in relation to the bottom bracket and has a relationship with a bike's reach.
• C. Seat tube length is the distance from the middle of the bottom bracket to the top of the seat tube. This length determines how high or low the bike's saddle can be set, and therefore how long or short any given rider's legs can be.
• D. Down tube length is the distance between the centre of the bottom of the head tube and the centre of the bottom bracket. Down tube length figures aren't usually quoted on manufacturers' size charts, but it's an easy measurement for a consumer to do at home when comparing one bike to another.
• E. Bottom bracket drop denotes how far above or below the horizontal line connecting the centre of the axles the centre of the bottom bracket is.
• F. Bottom bracket height is the distance between the centre of the bottom bracket and the ground.
• G. Wheelbase is the horizontal measurement between the centre of the front and rear axle.
• H. Front centre is the horizontal length between the centre of the bottom bracket and the centre of the front axle.
• Reach (not pictured) is the length between the bottom bracket and the centre of the top of the head tube. Reach provides the best indication of how 'roomy' a bike will feel, especially when it's being ridden standing up. Click here to find out how reach and stack height affect each other.
• Rear centre/chainstay length (not pictured) is the horizontal distance between the centre of the bottom bracket and the rear axle.

• A. Effective seat angle is the angle of the line that connects the bottom bracket to the centre of the top of the seatpost when it's at pedalling height. Manufacturers often quote their effective seat tube angle, but the height at which it's measured isn't usually disclosed.
• B. Actual seat angle is the angle of the bike's seatpost measured from horizontal.
• C. Head angle is the angle of the fork's steerer tube measured from horizontal.

Getting the perfect mountain bike fit

Getting a bike to fit you perfectly is something you need to work at. We know riders who’ve ridden for years on what they thought was their perfect bike, with perfect reach, perfect saddle height, perfect handlebar shape, a perfectly set up fork and the best mountain bike tyres, perfectly inflated, until they discovered it wasn't.

They discover that a basic change, perhaps even a few basic changes, to that setup seems to make them ride better.

It'll often be something as simple as a different handlebar sweep, different tyre pressures or more or less suspension-fork sag. It’s often minor details of bike setup that change the way you ride and feel about your bike.

When we sit on a bike, we make contact in three places: our hands on the bars, our feet on the pedals and our backside on the saddle.

It’s the relative position of these three areas that governs how the bike fits and several variables influence their exact location: top tube length, seat angle, distance from bottom bracket to saddle, crank length, bar height and width, stem length and saddle angle all play a part.

Tweak your ride setup from time to time, then give yourself a few rides to decide whether you like it or not. There are some things that feel wrong when you first change them, but right after a few rides.

In the following sections, we’ll lay down the basic guidelines of bike fit, together with variations to consider and the reasoning behind them.

Don’t think of a bike fit and setup as something that’s carved in stone, though. Use our guidelines as a starting point, then go out and experiment.

Seat tube length and standover

When a bike is listed as ‘X’ inches, what does that mean?

In most cases, it’s the distance from the bottom bracket axle to the top of the seat tube, but it can be to the middle of the top tube or to various other places – there isn’t a universal standard.

Even if there were, it would be no indicator that you could straddle the top tube and clear it because top tube shape and bottom bracket height vary substantially.

This all relates to ‘standover height’: an important aspect of any bike fit, since it governs the clearance of your crotch!

The seat tube should leave you with an acceptable standover gap – the distance between the top tube and your crotch – and usable standover clearance.

To get this, stand back as far as you can while over the bike and ensure there's a minimum of an inch of room from the top tube to your crotch area.

If you adhere to this advice then your frame should provide you with a large range of adjustment at the seatpost, which is important for finding your optimum mountain bike seat height.

While this holds true for beginner and cross-country bikes, depending on the shape of the frame the rules change slightly, for example if it has a low-slung top tube or if you're looking to buy a downhill or enduro bike – which have different geometry entirely.

It's therefore important to not use seat tube length and standover height as the only measure of a bike's fit. It also highlights how crucial sitting on different sizes of your prospective purchase is.

Saddle height and crank length

The majority of 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.

Similarly, if you’re long-legged you may benefit from a longer crank so you can make the most of your lofty dimensions.

For general trail riding, aim to set saddle height on your bike for maximum power and efficiency. Too high and your hips will rock from side to side, wasting energy; too low and your muscles won’t deliver power effectively.

Adjust the saddle height so when your heels are on the pedals at the bottom of the pedal stroke, your leg is fully extended – this means when you move your feet to the right position, your knee won’t lock out.

If you need more clearance, drop your saddle an inch or two.

Top tube length and reach

Another important consideration is the top tube length. Together with seat height, stem length and handlebar position, top tube length dictates the comfort and efficiency of your body on the bike.

To confuse matters further, the aspect of top tube length that matters is not the top tube itself, which often slopes, but the reach number.

Although top tube length will give a good indication of how a bike will feel when you're seated, the reach figure is most relevant for when you're standing up and is particularly pertinent to descending, but also helps contribute to the feel of a bike's size.

A cross-country rider may prefer a long, stretched-out position, but a beginner who has never taken a bike off-road may want to be more upright for extra comfort, with less weight on their hands and wrists.

Your reach is often a compromise between comfort, control and pedalling efficiency.

Find what works best for you, but avoid being too hunched or too stretched out, since this can cause discomfort and back problems.

Seat angle and effective top tube length

The cranks (or bottom bracket) are never situated directly below the saddle, and for good reason. If they were, you’d be placing excessive weight on your arms to support your upper body when you lean forward.

Thus the seat tube lies at an angle, which determines how far behind the bottom bracket the saddle will be and how you’re balanced when seated.

Too much can be counterproductive, but luckily the range of angles is usually quite narrow, so this measurement isn’t normally that important.

If we take two bikes with the same ETT length but different seat tube angles, the slacker-angled machine will have a bottom bracket that’s further forward in relation to your saddle and vice versa.

The upshot of this is you can have two bikes with the same reach that handle differently, due to how they distribute your weight.

One of the biggest mistakes made by beginners is to slide the saddle too far back. While it may be psychologically reassuring to sit well back from the ‘attacking terrain’ position, too little weight on the front of the bike can make the steering feel vague and stop your suspension fork from compressing efficiently.

Sit further forward and you’ll get maximum use of the fork, full use of the front tyre tread and the bike will handle better.

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

Foot position and cleats

The best mountain bike pedals can be flat or clipless. With flat or platform pedals, the ball of the foot usually drops into a comfortable position above the pedal axle.

However, clipless pedals can be more problematic to get right, so it's essential to know how to set up clipless pedal cleats. A good place to start is to find the ball of your foot and place the cleat directly underneath.

Once you’ve found this spot, adjust back and forth – minor changes can affect which muscles are utilised and how effectively you pedal.

See what works best for you. Lateral positioning is a personal preference: a narrower stance can improve efficiency, but be careful that your shoes don’t hit the cranks during the pedal revolution.

The angle of the cleats should match the natural angle of your feet, which you can see easily if you use flat pedals.

Many of the latest clipless pedals have built-in float, which helps your foot achieve a natural angle and is a good option if you’re unsure what’s right for you.

Experiment with the final setup; once you have this sorted, the pedal stroke will feel fluid with no twisting of the ankles, knees or hips.

This can take a few rides, but is worth persevering with – when you hit that sweet spot, draw a line around the cleats for reference when they need replacing.

Problems caused by the wrong size mountain bike

If you do end up on the wrong-sized mountain bike, whether that's too small or too large, you could end up not enjoying yourself as much or, as a worst-case scenario, getting injured.

Aches and pains can be caused by aspects of bike setup, but also by other things, so don’t take this list as gospel; it’s a rough guide.

See your doctor if something is really hurting, especially if it continues to be painful after riding and it’s not solved by the adjustments mentioned here.

Be aware that a lot of your aches and pains on a bike are simply caused by insufficient muscle support. In other words, you may just need to ride more and do some core muscle training to work things out.

Here are some common ailments and their causes:

• Knees: Knee pain when riding can be caused by your saddle being too high or too low, or your shoe cleats being poorly adjusted. Some riders find that a pedal/cleat system with more free float gets rid of knee pain.
• Back: Back pain during/after riding will often be related to poor core muscle support, so there may not be a quick and easy setup fix. But try changing the position of your handlebars and/or your reach from the saddle to the bars. We know a lot of riders who’ve solved lower-back pain simply by putting the stem up or down by half an inch, or getting a handlebar with more backsweep. Back pain can be indicative that your bike's frame is the wrong size.
• Shoulders/arms/neck: We're putting these three together because it’s often similar aspects of setup that cause aches and pains in these areas, namely too much stress being placed on these bits of your body. This may be caused by being sat too far forward on the bike, but it can also be down to sitting too far back, making you curl your shoulders and preventing you holding the bar properly. Experiment with stem height and saddle-to-bar reach. Try different bar shapes: a lot of riders find that more backsweep or upsweep on a bar will make them feel far more comfortable. Also try anatomically shaped grips, which support your hands better. If your shoulders, arms or neck are hurting, they could be telling you that your bike is either too big or too small.
• Hips: A lot of hip problems among cyclists are caused by the saddle being either too high, too low, tipped too far back or forward, or not offering the right sort of padded support.

When your bike is the correct size:

• Arms: Good bike position results in relaxed shoulders and slightly bent elbows.
• Saddle: Correct saddle position is essential for balance, control and pedalling efficiency.
• Knees: Having very slightly bent knees at the bottom of each pedal stroke is perfect.
• Frame: Getting the correct frame size is essential, but it’s only a starting point for perfect bike setup.
• Shifters and brake levers: Don’t just leave them in one position. Experiment with setting them further in on the bars or tilting them.

Although everyone is different – some folks may have longer legs and a shorter torso, while others may have long arms and short legs – starting with the correct-sized frame allows you to further tune the position using stem, bar, seatpost and saddle tweaks.

How to get the perfect mountain bike fit

Follow these tips on how to adjust your bike so that it fits you better.

Mountain bike saddle height

When answering the question 'what size mountain bike do I need?' what really matters is how the bike feels when you sit on it and ride.

The first thing you need to do, in the shop or on a demo ride, is set the saddle at the right height. If you can't get the saddle to a comfortable height then the bike you're riding could be the wrong size.

A rough approximation of maximum seat height for efficient pedalling is your trouser leg measurement plus 13cm (5in) from the centre top of the saddle to the centre top of the pedal.

To work it out more accurately, with comfort and efficiency in mind, sit squarely on your saddle with the cranks in a straight-up/straight-down position.

The saddle is at the right height when your heel just touches the top of the lower pedal with your leg straight; your crank should be right at the bottom of its stroke.

If you have to tilt to one side on the saddle to achieve this position, then the saddle is too high.

Place your foot on the pedal in the ready-to-pedal position. If your leg is straight with your heel on the pedal, it should be slightly bent at the knee in a pedalling position. You should never feel as if you’re being forced to rock your hips from side to side on the saddle while pedalling.

You may need to make adjustments to this position according to confidence and comfort preferences, and depending on what shoes you wear.

Keep in mind this is all based on efficient pedalling for cross-country trail riding. You will want to set your saddle lower for difficult descents – a job taken care of by dropper posts.

Mountain bike saddle position

As a rule, start with your saddle as level as possible on the top. This is an efficient cross-country position, but some riders will prefer a slightly tipped-back saddle for tricks, stunts and/or steep downhill work.

A few will prefer the nose of the saddle slightly tipped down for climbing or a more forward-biased ride posture – or if your bike's seat tube angle is particularly shallow. But dead flat is right for most riders.

Saddle rails have a lot of fore/aft slide adjustment, and not all seatposts are created equal.

Some have set-back clamps, others have clamps in line with the top of the post. This has a bearing on the position you’re trying to achieve with your saddle.

Set the saddle too far back and it'll make the bike feel back-heavy, possibly even too light at the front, especially when you're climbing.

A saddle set too far forward can cramp your ride position and make you feel as though you’re putting too much body weight on the front of the bike.

In theory, if a bike has exactly the right reach for you, and the fork is set up properly, you’ll probably end up with the saddle set dead centre on its rails. If you’ve got long arms for your height, you may end up with the saddle set well back: short arms and you’ll be looking at inline seatposts and your saddle forward.

You can use stem length and handlebar position to fine-tune the way you sit over the bike too.

How far away should the handlebars be from the saddle?

If you have access to different stem lengths and different-shaped handlebars, experiment with different ride positions, adjusting your saddle accordingly.

Arm, leg and torso length will vary between riders of the same height, and body weight distribution can have a major bearing on setup.

A starting point to work out the correct saddle to handlebar reach for XC or light trail bikes is to put the tip of your elbow on the nose of the saddle and see how far your longest finger reaches along the stem; forearm length is generally a good indicator of full arm and torso length.

Most riders looking for a fast and efficient trail riding posture will discover their longest finger reaches to almost exactly halfway between the top of the steerer tube and the handlebar centre.

You can fine-tune ride feel from that point by adjusting your seat position, stem length and height, and handlebar type.

Some handlebars have a more generous backsweep than others, and you can turn bars in the stem to tune your hold position/wrist angle. We know riders who like their bars straight and others who find a 25-degree backsweep their ideal solution.

Keep that elbow tip to finger tip measurement in mind when working out whether a test bike is the right size for you.

It's also worth considering the type of riding you're going to be doing because while this rule of thumb works for disciplines where you spend a lot of time pedalling seated, it's less appropriate for hardcore trail, all-mountain, enduro or downhill.

Mountain bike handlebar height

How high you have your bars is a function of stack height, steerer tube length (and the number of adjustment spacers on it), stem height and rise, and handlebar rise.

Some riders feel relaxed with their bars at roughly saddle height, others (particularly cross-country racers) have them way below saddle height to achieve a flat-backed streamlined posture on the bike.

Control positions

Brake levers and gear shifters can be put in different positions on the bar. On most brakes, you can adjust lever reach too, and on some you can adjust the point of contact where the brakes compress the pads.

We know riders who put up with their thumbs rubbing on their gear shifters for years before realising that setting them half an inch further inboard on the bars solves the problem without making them harder to use.

Bar width can be trimmed too: cutting an inch off either end of a handlebar might make a difference to your ride comfort.

Equally, you might benefit from better control with a wider bar. Swivelling bars a few degrees back or forth in the stem can also make a difference.

Don’t be afraid to try something different, but try it for a few rides in order to find the pros and cons of a new setup.

Components that affect comfort and control

Tyres

Tyre compounds, as well as pressures, will affect the way a bike feels on the trail. Cleverly treaded dual-compound tyres with a high TPI (threads per inch) carcass construction will generally deform more over rough terrain, and so grip better, without any increase in rolling resistance.

Cheaply made tyres tend to grip less and are more prone to losing traction when under pressure, especially in wet conditions.

Grips

Soft or sticky compound handlebar grips, or grips made from soft foam, might not be as hardwearing as others but they’re far more comfy, absorbing vibration and making you feel more at ease on the bike on rough terrain.

Saddle

The right sort of surface material and the right sort of padding on a saddle is obviously going to make a huge difference to the way you feel about your bike.

As a rule, you should be able to move easily on the surface of a saddle; fancy embroidered graphics aren’t always conducive to this. And don’t assume that more padding is always better.

Slimline saddles with minimal padding in just the right places are often more comfy than big bouncy affairs, which will often chafe after a while.

Pedals

The efficiency of your pedal/shoe interface has an impact on how you ride.

Stiff-soled shoes with inset cleats fixed to clipless pedals will make you a more efficient ‘full circles’ pedaller. But read instructions carefully when it comes to cleat position because poorly positioned cleats can cause problems, especially with knees.

Most riders start with their cleats set dead centre in the shoe recess, but that doesn’t feel right for everyone, and some cleats/pedals offer more free float movement than others.

Tyre and suspension setup and pressures

Your tyres, suspension fork and rear shock effectively provide an adjustable cushion between your bike and the ground.

Being able to set up your mountain bike suspension properly underpins your overall control and comfort. The same applies to mountain bike tyre pressure.

Big-volume tyres can be run at lower pressures than small-volume ones, and big-volume tyres with a low knob profile will often roll just as fast as, and offer more comfort and control than, skinny tyres.

Tyre pressures need to be adjusted to suit your riding style, the width of the tyres fitted to your bike, the width of the rims the tyre's seated on and the conditions or trail type you're riding on. And of course, it depends if you're running tubeless tyres. There is no universal rule.

However, if you feel your bike is getting pinged around on the trail and you've verified your suspension is set up correctly, then your tyres might be too hard.

Equally, if it feels as though your tyres are squirming or rolling on the rim in turns, then they could be too soft.

Fork and shock settings will vary according to make and model, so consult manufacturer guidelines to find out how you should be dialling in your suspension.

Get an expert opinion from a bike fitter or reputable bike shop

Even if you feel overwhelmed after reading all of this information, fear not. Any reputable bike shop will be able to help you with finding the right size mountain bike for your needs.

Some shops even offer a professional bike-fitting session to really help you nail down what size bike you need and how you can modify your current bike to fit you better.

Don't be afraid to head to your local bike shop to find out more.

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