Our Ultimate Mountain Bike Size Guide

In the same way that human beings come in all shapes and sizes, so do mountain bikes.

Choosing the right mountain bike frame size can be tricky. Getting it wrong can lead to injury or handling issues. Get it right, though, and it can bring years of enjoyment on two wheels. Being comfortable will inspire more confidence and boost your ability to ride for longer and over trickier terrain.

Before looking at sizes, it is worth thinking about the type of riding you intend to do and decide which type of MTB would be most suited to you.

Check out our MTB size guide below.


  1. Our mountain bike frame size guide
  2. How to measure a mountain bike frame
  3. The difference between road and MTB sizes
  4. How geometry affects the ride
  5. How components are scaled
  6. More on fit
  7. How to choose the right wheel size
  8. Tips for a great fit

1. Our mountain bike frame size guide

Below is a very approximate mountain bike frame size chart using height and inside leg measurement.

Size Seat Tube Size (Inches) Height Range (Approx) Inside Leg
XS 13”-14” 5’-5’3” 28”
S 14”-16” 5’3-5’7” 29.5”
M 16”-18” 5’7”-5’10” 31.5”
L 18”-20” 5’10-6’1” 33”-35”
XL 20”-22” 6’1”+ 36”+

2. How to measure a mountain bike frame

Manufacturers have made buying mountain bikes which are the correct size relatively straight forward. Most manufacturers provide a sizing and geometry chart for each specific model. Sizing and geometry charts are sufficient for the vast majority of riders to pick the right size bike for them.

The two key frame measurements on the geometry chart are seat tube length and top tube length.

  • Seat tube length — This can affect how much stand-over clearance there is between you and the bike. This is important for new riders as you may well need to stop and get your feet on firm ground quickly. However, some bikes — such as those with longer suspension travel — can have zero standover clearance, and still be the correct size.
  • Top tube length — This determines the comfort, fit and handling of your bike to a large extent. Ideally, you should aim not to feel either cramped or stretched.

3. The difference between road bike and MTB sizes

For road bike riders looking at MTBs, sizing can be confusing because MTBs tend to have a smaller seat tube and longer top tube length than their road bike cousins.

As mentioned above, manufacturers assist customers through simplifying the sizing process using the ‘catch-all’ sizing (Extra Small through to Extra Large). For more details, see the example chart above.

You can read our in-depth road bike size guide here.

4. How MTB geometry affects the ride

MTB geometry is important. The angles at which the tubes are joined to form the frame have a profound impact on how the bike will ride.

The ideal geometry largely depends on what type of riding you do. Cross-country-style MTBs tend to have steeper geometry (for example, they may have a 73.5-degree seat tube and a 71-degree head tube) whereas enduro and trail bikes, tend to have slacker geometry (for example a 71-degree seat tube and 67-degree head tube).

5. How components are scaled

Manufacturers scale components to fit average riders within each sizing bracket. For example, an XS size mountain bike with be likely to have shorter 170mm cranks and a shorter stem because riders around 5 foot tend to have shorter legs and a smaller upper body.

Alternatively, an XL mountain bike may well come with longer 175mm cranks and a longer stem, because, on average, taller riders have longer legs and a longer torso.

6. Getting the right fit for your body shape

The vast array of MTB styles means that the general sizing that manufacturers use (small, medium and large) is ideal. It virtually does away with the need for buyers to have to consider reach and standover height.

Sizing and fit are largely dependent on the style of bike. So a medium sized rider on a medium sized 100mm front suspension cross country bike, for example, might have an inch standover room, between them and the top tube. On a medium Enduro style bike with 160mm suspension, they may not have standover at all. Unlike road riding where a lot of time is spent riding in the same position, Mountain biking encourages lots of changing of position – so bike fit demands less fine-tuning.

The only time that general sizing doesn’t quite work is with unusual limb or core sizes. For example, someone with a very long torso and short legs, might struggle to find a good fit with general sizing. However, getting a longer or shorter stem and adjusting the saddle position can usually dial in the perfect fit.

7. How to choose the right wheel size

While wheel size doesn’t necessarily affect bike fit, it is important to know the effects wheel size will have.

26” wheels

The original wheel size for MTB is strong and flickable. 26-inch wheels boast fast acceleration but are easily slowed when rolling over bumpy terrain. While there are plenty of 26-inch-wheeled bikes still around, 27.5-inch wheels have become the new standard. 26-inch wheels have largely been assigned to the MTB history books.

29er wheels

29er wheels are the largest diameter MTB wheel and are best for fast rolling and getting over obstacles, they also hold momentum well. 29er wheels can be a little ungainly for smaller riders.

27.5/650b wheels

27.5 / 650b wheels are a little smaller than 29ers and are a bit more manoeuvrable or ‘chuckable’ out on the trails. These wheels offer the best mix of ride characteristics for the majority of riders.

Plus-size wheels

Plus-size wheels, such as 27.5 (+) plus allow fatter tyres (up to 3.0″ wide) which can be run at lower pressures and can offer more grip. Plus size wheels can only be used on a (+) Plus specific frame & fork. Fat Mountain Bikes use 26” wheels with large volume tyres.

8. Tips for a great fit

Here are our top tips to ensure you always get a great fit when choosing your mountain bike:

  • Aim for a relaxed, comfortable shoulder position.
  • Ensure you need to lean forward to grasp the handlebars. If your back is almost vertical, the bike could be too small.
  • Similarly, if you feel overstretched, the bike is probably too large for your frame.
  • Have a friend hold the bike while you sit and pedal backwards. Does this seated position feel comfortable?
  • Ask for a test ride. Even a short one should confirm whether the reach is about right for you.
  • If you need to, adjust the saddle rail position (most saddle offer around 30mm adjustment) and the stem length or angle, too. Adjusting your saddle can help to dial in the perfect fit, as long as the bike is not too small or too large.
  • There are no ‘hard and fast’ guidelines regarding fit.

Ready to ride

This MTB size guide will stand you in good stead, but if in doubt, feel free to give us a call on 01772 432431, or email sales@merlincycles.com, and we’ll help you out.