How to Choose the Right Size Mountain Bike in 2023

Every spring there is an influx of riders keen to get into mountain biking, and at Pinkbike and Outside, we know that these newcomers are often overwhelmed with information and opinions. So, we’re launching a seven-part series called MTB Explained, where we help new riders navigate some of the basics of our sport. If you’re new, welcome to the best damn sport in the world, and if you’re a long-time rider let’s welcome these folks to the club.


Picking the right frame size is one of the most important decisions you'll make when buying a new bike. The frame size affects the comfort, agility, stability, and all-around handling of the bike - arguably more so than the differences between two comparable models from competing brands. Here's some advice that's been repeated a thousand times: don't be tempted by a great deal on a bike that's not the ideal size.


Check the chart

So how do you pick the right size? Luckily, these days it's usually incredibly straightforward. Go to the website of the bike you're looking to buy, click on the geometry/sizing sectiont and check which size the manufacturer recommends for your height. That's it. It may sound too easy, but these days manufacturers have got to the point where sizing recommendations are a good guide for most people.

But there are two potential problems with this: what if you're on the border between two size recommendations on the manufacturer's chart, and what about older (secondhand) bikes?


What if I'm in between sizes?

If your height puts you in between two sizes, there's a judgment call to be made. You should be able to ride either without any problems, but in general, sizing down will give you a more lively ride that will suit tighter terrain, lower speeds, and a more playful riding style; sizing up will offer more stability, with less chance of pitching forwards when braking or riding over bumps, which suits faster, rougher terrain or a "to the point" riding style.

In the above video, I compare two sizes of the same bike in terms of riding feel and against the clock. For me, there was no real difference in how fast I could ride and it's surprisingly easy to adapt to either, but one thing I noticed is that the bigger size made more sense once I knew the track and was up to speed. So if you usually ride trails that you know well, that may tip the balance towards sizing up, whereas if you're often exploring new trails the agility of a smaller size might make more sense.


Note that reach is purely a measurement of the frame - it doesn't take into account the length of the stem, the rise of the handlebars, or the number of spacers under the stem. All of these will affect how big the bike feels to ride so make sure the bikes you're riding have the same stem length as what you would run on the bike you're intending to buy and a bar height that's appropriate for you.

Also, the reach on its own doesn't tell you how far the handlebars sit in front of the saddle; this also depends on the effective seat tube angle, which is the angle of a line drawn from the bottom bracket to the top of the seat post. Seat tubes have got much steeper in recent years, which moves the saddle forwards toward the handlebars. This means a modern bike with a long reach doesn't feel very long or stretched out when sitting down. The effective top tube length - that's the length of a horizontal line drawn from the top of the head tube back to where it meets the seat post - is arguably a better measure of how roomy the bike will feel while seated. But because the saddle can be slid back and forth on the saddle rails to adjust the saddle-to-bar distance (while the reach is fixed) it's usually better to focus on reach when choosing a bike.


What about older bikes?

If you're shopping for an older bike - by which I mean one made before about 2015 - the above doesn't necessarily apply.

In the early days of mountain biking, sizing was almost entirely based on the seat tube length, or the distance from the bottom bracket to the top of the seat clamp (usually measured in inches). Taller riders needed a taller frame in order to get their saddle high enough and shorter riders needed a shorter frame in order to get the seat low enough, but the horizontal length of the bike (which is critical for stability and handling) was almost an afterthought and barely changed between the smallest and largest sizes.

Throughout the history of mountain biking, this gradually shifted to the point where now, long and travel-adjustable dropper seatposts make it possible for most riders to fit on a range of sizes without worrying about the seat tube length at all. But if you're looking at buying an older bike, most experts would now agree that the reach and wheelbase are going to be on the short side (especially for taller riders), making for handling that would be more fairly described as "sketchy" or "terrifying" rather than "agile" or "fun".

So in this case, it's usually worth sizing up if you can. For example, at 191 cm or 6'3", I'm choosing an Xl or sometimes a Large with modern bikes, but with bikes from a few years ago I'd be looking for an Xl or XXL. The limiting factor for sizing up is usually the seat tube length - this is because older bikes have long seat tubes and short reach numbers.

To work out if the seat tube is too big, use a bike where the saddle height is at the right height for pedaling, then measure the distance from the saddle rails to the center of the bottom bracket. Now subtract the seat tube length of the bike you're thinking of getting; the number you're left with is the "collar-to-saddle rails distance", which is the room that's left for a dropper post. You can then plug this number into this calculator from OneUp to find out what's the longest dropper post that will fit.

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