How to Find the Right Size Mountain Bike

If you’re in the market for a new mountain bike, or looking to upgrade your ride, getting the size right is just about the most important thing. You’ll want a bike that feels comfortable, is responsive and is safe to ride.

If you want your rides to be fast, fun and safe then read on.

Mountain Bike Size Chart

If you’ve checked out our range of mountain bikes, you’ll see that mountain bike frame sizes are listed either in incher, or as a range of sizes. You can find mountain bike frames in extra small (XS), small (S), medium (M), large (L), extra-Large (XL) and extra-extra-large (XXL) sizes to suit riders of all shapes, sizes and styles.

This simple MTB size guide should provide you with a rough guide for which frame size would suit you. The only measurement you need to know is your height.

We recommend that you use this chart to help you narrow down your search. There is still more to consider when finding the right bike for you. Let’s delve into the dirty details.

Mountain Bike Types

If you’re searching for a new bike, then check out our mountain bike buyer’s guide and electric mountain bike buyer’s guides. You’ll find them packed full of essential information that you need to know.

All modern mountain bikes will have suspension at the front, with many having suspension at the rear too. Bikes with only front suspension forks are called hardtails, and those with rear suspension and front suspension are called full-suspension bikes.

Both front and rear suspension work to smooth out lumps and bumps. When you go over an obstacle, the suspension compresses, absorbing the shock.

Your choice of mountain bike frame size and whether it’s hardtail or full-suspension depends on your chosen riding style. How you ride your bike and where you ride your bike will have an impact on frame sizing. Downhill riders prefer full-suspension bikes with large frames to roll over obstacles, whereas XC riders might typically opt for smaller frames that are quicker to handle over, under and around obstacles.

Here’s a brief introduction to some of the different types of mountain biking:

• Cross-Country or XC - XC can involve anything from a gravel ride to a run at the local trails centre (and everything in between). Hardtail or full-suspension? It’s up to you.
• All-Mountain - All-Mountain riding means actually riding on mountains. It includes lots of climbing, tricky technical sections, jumps, lumps and bumps, so we recommend a full-susser.
• Trail riding – Trail bikes are engineered to deal with the fast and varied terrain you’ll find at the UK’s trail centres. You can ride either a hardtail or full-suspension bike.
• Downhill - Downhill bikes are designed to do one thing: go downhill quickly. Full-suspension is preferred here.
• Enduro - Enduro bikes are built to tackle anything and everything, including riding up mountains and back down them. Anything goes.

If you’re new to mountain biking, we reckon that a cross-country or trails bike is probably the best bet. These are built to tackle trails and off-road rides, perform well on gravel and even on the road. You can find a range of affordable XC bikes at Halfords.

We also stock a range of electric mountain bikes (EMTBs). These amazing machines are transforming what’s possible on the trails. While they’re packed full of innovative tech, they’re reassuringly familiar with frame sizes and components the same as their pedal-powered cousins.

Mountain bike measurements

Finding the right mountain bike size is all about safety. You need to be able to control the bike, steer safely and avoid obstacles. Here are some mountain bike fit fundamentals:

• There should be a gap of around 2 inches (approximately 5cm) of clear air when you are standing over your frame with your feet on the floor. (This is a basic guide, and doesn’t work for all riding styles, such as downhill riding.)
• Pedalling should be comfortable, with a slight bend in your knee at the bottom of each stroke.
• You don’t want to feel cramped up or stretched out on the bike. You should feel comfortable and relaxed, ready for hours in the saddle, with the freedom to cruise in the saddle and climb out when needed.
• You must be able to reach and operate both brake levers.

Mountain bike sizes are measured by the length of the seat tube, so the best place to start is with your old bike. Simply measure the length of your current bike’s seat post from BB to seat clamp and buy the same sized frame.

If you don’t have a mountain bike, then it’s time to grab a tape measure. By taking your inseam measurement, you can get a more accurate frame size guide. Here’s how it works:

Inseam measurement x 0.67 – 5 inches = your ideal frame size.

This is a bit more advanced than using your height and provides a more reliable and scientific approach to mountain bike sizing. It’s not going to be perfect, but it can help you narrow down your selection of models.

How to measure a mountain bike frame

Mountain bike sizes are typically measured according to the seat tube length, but there’s much more to getting a bike that fits you like a glove. A professional bike fitter will take into consideration all these measurements when finding a frame that fits. You may not want or need, a professional bike fit, but it can be useful to understand some of the standard terms used.

If you’re handy with a tape measure, here’s how to measure a mountain bike frame.

• Seat-tube length – the distance between the bottom bracket (crank) and the top of the seat tube (not including the seat post and saddle).
• Wheel size – the size of the wheels (usually 27.5” or 29”, but some older bikes have smaller 26” wheels).
• Effective top-tube length – the effective top tube length is the distance between the top centre of the head tube back to the centre of the seat post.
• Reach – reach is the distance between the centre of the head tube to a line drawn vertically from the bottom bracket. It’s a more reliable indicator or reach than the effective top tube length.
• Head angle – the head angle is the angle of the head tube and the ground. A shorted head angle makes a bike more agile and responsive on trails, but can feel skittish downhill or at speed.

Other considerations, such as seat angle and chainstay length, can affect how a bike feels and performs, but are less critical when selecting a frame size that fits.

Sizing Up Or Sizing Down Your Mountain Bike

You may find that you’re stuck between choosing a larger mountain bike frame or a smaller one. So how do you select the correct sized mountain bike?

If after measuring your height and inside leg measurements, you’re stuck between two sized mountain bikes, then your reach can be a good deciding factor. Your reach is influenced by your upper body length (also known as your ‘trunk length or sitting height).

One way to narrow down your search is by measuring your Ape index. Your Ape index is essentially whether your arm span is wider than your height or not.

To find your Ape index, stretch out your arms and get someone to measure the distance between your two most extended fingers. Now, subtract that from your height. If your arm span is greater than your height, you’ve got a long reach, so we’d recommend the bigger frame. If it’s less, we would advise you to choose the smaller frame.

While the numbers can help you narrow down your search, it’s not always conclusive. Some riders prefer a larger frame, and others a smaller one. Here’s what you get for sizing up or down.

What You Get for Sizing Up

A larger mountain bike frame means a longer wheelbase (the distance between the two wheels). The riding position will be more stretched out, which means you’ll bounce over obstacles, rather than flicking or whipping your way around them.

What You Get for Sizing Down

Some riders find smaller mountain bike frames to be more manoeuvrable and faster. The shorter wheelbase can make it easier to navigate obstacles. It’ll be slightly lighter than the larger frame. The riding position will be more upright.

If you’re looking to fit a dropper seat post, we’d recommend a smaller frame.

How can I set up my bike for my riding style?

Finding the right-sized mountain bike is just the start in getting the perfect bike fit.

If you’re planning longer, more relaxing mountain bike rides, you may want to adjust your bike, so you’re in a more upright position. You can adjust your saddle with a hex key or multi-tool to a 30-degree angle between your hip, knee, and ankle. This will be more comfortable over a longer ride.

Looking for more speed? For a more aggressive riding position, try angling your seat forward a little. This will give you a little extra power on the pedals.

Beyond minor adjustments, there is a whole world of aftermarket parts and upgrades that can help to personalise and perfect your bike. Whether you’re looking for a new set of pedals, an upgraded wheelset or different sized stem, then you’ll find a whole host of awesome accessories and upgrades at Halfords.

Demoing Mountain Bikes

Finding the right mountain bike frame size is more than just numbers, it’s about the right feel. We know it’s not always possible to visit a Halfords store, but if you can, you’ll be able to view, sit on and even test-ride a selection of the UK’s best mountain bikes from brands such as Carrera, Voodoo and Boardman.

If you visit a Halfords store, one of our trained technicians can provide. Information and advice on getting the best mountain bike fit. They can explain the differences between bikes and help you to find something that looks good feels great and rides fantastically.

MTB Sizing - Frequently Asked Questions

Here are some quick answers to popular questions around MTB sizing.

How are mountain bikes sized?

Mountain bike sizing is done in inches, but you’ll actually see frames listed according to their size (extra small, small, medium, large, extra-large and extra-extra-large). Our Halfords Mountain Bike Size Guide provides a straightforward way that you can use your height to help you find the best mountain bike frame size for you.

Which MTB wheel size is best for me?

You can find mountain bikes with 27.5” (650b) and 29” wheels and each has its own advantages and disadvantages. (Older mountain bikes also used to come with 26”, We’re not going to go into much detail here, but bikes with 27.5” wheels can feel more responsive and manoeuvrable. Bikes with bigger 29” wheels will roll better and provide greater stability. Ultimately, the choice is yours and, as always, we recommend you try before you buy (if possible).

What size mountain bike do I need?

The correct sized mountain bike is one that feels comfortable to ride, keeps you in control and is – most importantly – safe. We’re all individuals, but by following our mountain bike size instructions, you should have a pretty clear idea whether you need a medium mountain bike or a large frame mountain bike.

Need some more help finding the right mountain bike for you?

If you need a hand at finding a mountain bike frame that fits, then visit your local Halfords store where you can chat with one of our bike experts. We’ll be more than happy to help you find the right frame size and offer guidance on how it can help personalised to provide the perfect ride. We can even build it for you.