Mountain bike sizing: what size bike do I need?

Use our guide to help you determine your perfect size of mountain bike

Choosing the correct size of bike is one of the most important decisions you’ll make. Don’t buy until you’ve read our advice so you can get the perfect mountain bike setup for maximum comfort and a reduced chance of injury.


A bike that fits correctly is a joy to ride, while one that’s too small can cause handling problems and be uncomfortable on longer rides. Read on for some advice on what frame size to go for, 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.

Anatomy of a mountain bike

We all come in different shapes and sizes, and so do most mountain bikes, so we recommend using the information below as a starting point and a guide. First, it’s good to know the anatomy of a mountain bike, as we’ll be referring to these some of these terms.

This diagram shows the different tubes used on an average mountain bike frame

Frame sizes

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 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. The two main measurements to consider when looking at frame sizing are the seat tube length, and the top tube length.

Seat tube length and standover

The seat tube should leave you with an acceptable standover gap (see pictures below) 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 saddle height.

Top tube length and reach

Another important consideration is the top tube length. Together with seat position, 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 a horizontal line from the middle top of the head tube to the middle of the seatpost.

You can see the handlebar stem and head tube clearly with this inset shot

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:

  • XS: Bike size 13-14in: generally for riders between 5ft and 5ft 4in
  • S: Bike size 14-16in: generally for riders between 5ft 4in and 5ft 7in
  • M: Bike size 16-18in: generally for riders between 5ft 7in and 5ft 10in
  • L: Bike size 18-20in: generally for riders between 5ft 10in and 6ft 1in
  • XL: Bike size 20-22in: generally for riders over 6ft 1in

(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 on this.)

Frame size problems

Two things to watch out for: the length of the handlebar stem and the standover clearance. Too big a frame? Could be painful – as this picture demonstrates.

Too big a frame leads to problems like this

Get the frame too big and you could suffer from:

  • A sore back from overreaching on long rides
  • A lack of standover clearance, leading to some particularly wince-inducing experiences
  • A lack of control of the bike
Too small a frame leads to problems like this

Get the frame too small and you could suffer from:

  • Injury on longer rides from a too cramped position
  • Potential toe overlap problems (where your foot clips the front wheel)
  • Too much standover clearance, leading to back problems when sitting down on longer rides

More fit adjustments

Ultimately, what matters most is how the bike feels when you sit on it and ride. If possible, get out there and test a few different frame sizes on a demo ride or two (many shops will let you do this) before committing to buy. Try different bikes at different places.

And as well as frame size, you’ll need to make sure your bike fits at all the main contact points too: saddle, handlebars and pedals. Once you decide on your frame size and have bought your bike, have a read of our mountain bike positioning article to make sure you get the perfect mountain bike setup.

You can fine tune the fit of the bike by altering some components like the handlebars and stem, but be aware that these alterations will also have an effect on how the bike handles – a longer stem isn’t the solution to a bike frame that’s too small.


For more detail on how to set up your MTB, check out our video on getting your handlebars and controls dialled in.