‘What size mountain bike do I need?’ A question frequently asked for good reason because choosing the correct size of bike is one of the most important decisions you’ll make.
We recommend you don’t buy a new bike until you’ve understood why it’s so important to get one that’s the right size for your height and body shape, and understand the changes you can make to your bike’s parts to help it perform better for your measurements, personal requirements and preferred discipline.
A bike that fits correctly and is set up properly is a joy to ride, making it easier to tackle trails faster with more control, but one that’s too small can be twitchy, nervous and be uncomfortable on longer rides, technical descents or just pootling along the flat.
Read on for some advice on what mountain bike frame size you should be conidering, especially if you’re in any doubt about it.
Looking for a guide to road bike sizing instead? That’s also something we can help with.
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. 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 that 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.
But 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.
Getting a bike that’s the perfect fit can seriously improve comfort, control and speed on the trail.Andy Lloyd
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:
13 – 14in
152cm – 162cm (5ft – 5ft 4in)
14 – 16in
162cm – 170cm (5ft 4in – 5ft 7in)
16 – 18in
170cm – 178cm (5ft 7in – 5ft 10in)
18 – 20in
178cm – 185cm (5ft 10in – 6ft 1in)
20 – 22in
185cm plus (6ft 1in plus)
(Bear in mind that road, cyclocross and hybrid bike sizes tend to be 3 to 4in bigger for riders of the same height – something that confuses a lot of riders when looking through bike listings. Read our guide to road bike sizes for more information.)
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, as we’ll be referring to these terms.
These key parts of a bike will define its geometry and how it fits.Matt Orton / Immediate Media
C. Seat tube
D. Top tube
E. Down tube
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.
The image shows you all the key length measurements you need to know when considering a bike’s size.Matt Orton / Immediate Media
A. Effective top tube length is the length of a virtual horizontal line drawn between the top of the bike’s head tube to 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 to the centre of the front axle.
Rear centre/chainstay length (not pictured) is the horizontal distance between the centre of the bottom bracket and the rear axle.
Seat tube and head tube angles are important to understand and can affect the rest of the bike’s geometry.Matt Orton / Immediate Media
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 effect seat tube angle but the height at which it is measured isn’t usually disclosed.
B. Actual seat angle is the angle of the bike’s seat post 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 perfectly inﬂated tyres, 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.
With the right size bike, you’ll instantly have more control.Steve Behr / Immediate Media
When we sit on a bike, we make contact in three places: our hands on the bars, our feet on the pedals and our bum 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 feel 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 actually 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!
Minimal standover height can have painful consequences.Oliver Woodman/Immediate Media
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 that 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 all together.
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.
Can crank length can make a difference to how your bike fits, especially if you’ve got long or short legs.Steve Behr
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, the saddle should be set at a position that helps you maximise 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.
Make sure your saddle is set at the correct height.Andy Lloyd
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.
The top tube length is responsible for more than just your comfort.Oliver Woodman/Immediate Media
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 that 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 half way between the bottom bracket axle and the rear wheel axle.
Foot position and cleats
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. 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.
Because women typically have different dimensions to men for a given height, the proﬁle of some models has been altered to give a better ﬁt, with some brands designing their bikes specifically for women based on body geometry data and rider feedback.
A bike that fits will be a pleasure to ride.Steve Behr
Look closely, however, and you’ll see that some brands use exactly the same frames as the men’s/unisex equivalent with just touches, such as the finishing kit, tailored for female riders.
Any one particular model won’t suit all women and some riders will ﬁnd that a unisex bike is a better fit.
Most have several things in common, such as thinner grips, lighter sprung forks, women’s saddles and narrower bars. But it isn’t just the ladies who can beneﬁt, men can also take advantage of this thinking and should check them out if they have small hands or narrow shoulders, for example.
Kids’-speciﬁc mountain bike fit and size
Many of the same rules apply for the young ’uns, whatever their age, but there are also a few extra considerations to be aware of.
The bikes are usually sized by their wheel diameter, from 12in up to 26in. As with an adult, top tube clearance and reach are very important.
Kids bikes come in all shapes and sizes to suit all ages and abilities.Black Mountain
However, unlike most adults it’s essential a child can reach the ﬂoor when seated for control and safety.
Never be tempted to buy a size up for ‘growing room’, it can be both uncomfortable and dangerous. Take a look at the controls: can they reach and operate them effectively?
It’s paramount that the brakes work well and can be easily squeezed with weaker hands. Take a look at the shifters too: are they struggling to push a thumb lever? If so, go for twist grip-type gears because many kids ﬁnd the ergonomics easier to handle.
Gears and cranks are also often overlooked, with a considerable number of bikes sporting adult length cranks and full-sized chainrings. Look for a good range of smaller gears instead, which will ensure they don’t struggle on the inclines.
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, injuring yourself.
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.
Knee, hip and back pain can ruin rides.Matt Grayson
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 set-up 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
Setting saddle height is crucial.BikeRadar
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 was 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.
And keep in mind that 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
Setting saddle angle is an often overlooked part of setting up your mountain bike.BikeRadar
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, and 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.
A saddle set too far back can 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 staring 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 that 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 to consider 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
Place spacers above or under the stem can totally alter how a bike rides.BikeRadar
How high you have your bars is a function of stack height, steerer tube length (and the amount 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 streamline posture on the bike.
Brake lever angle and lever throw and reach are also crucial elements to get set up correctly.BikeRadar
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.
The Minion DHF front tyre offers fantastic, predictable cornering grip.Alex Evans
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 make you feel more at ease on the bike on rough terrain.
SQlab claims that the raised rear helps to improve your grip on the saddle.Georgina Hinton
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.
Pedal preference is a personal thing, so figure out what you’re looking for and do your research before buying.Andy McCandlish
Tyre and suspension setup and pressures
Your tyres, suspension fork and rear shock effectively provide an adjustable cushion between your bike and the ground.
Setting them up properly is crucial to your overall control and comfort. 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.
Make sure you check your pressures before every ride.Steve Behr
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. There is no hard-hitting rule.
However, if you feel like 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 like 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 be sure to check out our suspension setup guide 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.
BikeRadar provides the world's best riding advice. We're here to help you get the most out of your time on the bike, whether you're a road rider, mountain biker, gravel rider, cycle commuter or anything in between. You can expect the latest news and features, in-depth reviews from our expert team of testers, impartial buying advice, how-to tips and plenty more.