Short answer: on a mountain bike, reach decides fit more than the size label. Reach is the horizontal distance from the centre of the bottom bracket to the top centre of the head tube, and it sets how stretched out you are when standing on the pedals. Size labels do not match between brands: a Kona Kahuna in Medium has a 440mm reach, while a 2026 Merida Big.Nine in Medium has 410mm. Use the brand's height chart to find a starting size, then compare reach against a bike you know fits.
Mountain bike sizing changed when dropper posts and long, slack geometry arrived. Seat tube length, the number road bikes are still sized by, now tells you very little about how a mountain bike will feel. This reference explains reach and stack, why the same letter means different bikes at different brands, how to choose between two sizes, and what you can adjust after you buy.
- Reach and stack explained
- Why seat tube length matters less
- Why size labels do not match
- Reach by rider height
- Choosing between two sizes
- Measuring the reach of a bike you own
- What you can adjust after buying
- Our mountain bike size charts
- FAQ
- Getting your size right
Reach and stack explained
| Measurement | What it is | What it changes |
|---|---|---|
| Reach | Horizontal distance from the centre of the bottom bracket to the top centre of the head tube | How stretched out you are, how much room you have to move, and how stable the bike feels at speed |
| Stack | Vertical distance from the centre of the bottom bracket to the top centre of the head tube | How high the front of the bike sits, and so how upright or low your position is |
Both are measured from the frame alone, so they do not change with the stem, handlebar or spacers fitted. That makes them the fairest way to compare two frames, even across brands.
Reach matters most on a mountain bike because so much riding happens standing on the pedals, on descents and technical sections. In that position the saddle plays no part, and the distance between your feet and your hands, which reach controls, decides whether you feel balanced over the bike or stretched and pitched forward.
Why seat tube length matters less
Road bikes are still often sized by seat tube length, as a 54 or a 56. Mountain bikes moved away from this for two reasons.
- Dropper posts. The saddle moves up and down while you ride, so frames now have short, straight seat tubes to make room for longer-travel droppers. Several sizes can share a similar seat tube length.
- Low top tubes. Sloping top tubes give plenty of standover clearance on almost every size, so standover rarely decides the size any more.
Seat tube length still matters in one way: it limits how long a dropper post you can fit. Shorter riders in particular should check that the frame leaves enough room to reach their saddle height with a dropper installed.
Why size labels do not match
There is no industry standard for what Small, Medium or Large means. Each brand decides its own reach for each size, and different types of bike within one brand differ too. Kona's published geometry for the Kahuna hardtail looks like this:
| Kona Kahuna size | Reach | Stack | Seat tube |
|---|---|---|---|
| Small | 415mm | 602mm | 370mm |
| Medium | 440mm | 611mm | 420mm |
| Large | 465mm | 620mm | 470mm |
| Extra Large | 500mm | 629mm | 530mm |
The 2026 Merida Big.Nine, an entry-level cross-country hardtail, has a reach of 410mm in Medium, which is shorter than the Kona's Small. Neither is wrong. The Big.Nine is built to be approachable and upright for newer riders, while the Kahuna has longer, more progressive trail geometry. But it means a rider who is a Medium on one may be best on a different letter on the other.
Brands' height charts also overlap differently. A 175cm rider falls in the Medium band on our Polygon chart (165cm to 175cm) and is between Medium and Large on the Cannondale Scalpel chart, where Large starts at 172cm.
Reach by rider height
As a rough guide for modern trail hardtails and full-suspension bikes, this is where most riders end up. Cross-country and entry-level bikes often run 20mm to 30mm shorter for the same height, and long-travel enduro bikes longer.
| Rider height | Typical reach on a modern trail bike |
|---|---|
| Under 160cm | About 390mm to 420mm |
| 160cm to 170cm | About 415mm to 445mm |
| 170cm to 180cm | About 440mm to 470mm |
| 180cm to 190cm | About 465mm to 495mm |
| Over 190cm | About 490mm and longer |
Treat these as a starting point, not a rule. Arm and torso length, flexibility and riding style all shift the right number, which is why two riders of the same height can choose different sizes. Shorter riders have the fewest options, and our guide to bikes for shorter riders covers them.
Choosing between two sizes
If your height sits in the overlap between two sizes, choose by how you ride:
| Choose the smaller size if you | Choose the larger size if you |
|---|---|
| Want a playful bike that is easy to throw around tight trails | Ride fast, open trails and want stability at speed |
| Are newer to off-road riding and want confidence | Do long days in the saddle and want room to move |
| Have a shorter torso and arms for your height | Have a longer torso and arms for your height |
| Ride lots of technical climbs and switchbacks | Ride steep descents where a longer wheelbase helps |
A quick way to judge your proportions is arm span against height. If your arm span is longer than your height, you will usually be comfortable on the longer size; if it is shorter, the smaller size.
Merida builds this choice into its naming on some models. Bikes using its Agilometer sizing, such as the One-Twenty, use labels like Short, Mid and Long with overlapping height ranges, so that a rider can pick the reach for the handling they want. Merida's example is a 175cm rider, who can choose Short for a more agile bike, Mid for a balanced one, or Long for more stability.
Measuring the reach of a bike you own
If you have a mountain bike that fits well, its reach is the best reference you have. Look it up in the maker's geometry chart for that model year and size. If you cannot find it, you can measure it closely at home:
- Stand the bike upright on level ground against a wall, with both tyres at normal pressure.
- Hang a plumb line, or hold a long spirit level vertically, from the top centre of the head tube to the floor, and mark the spot.
- Do the same from the centre of the bottom bracket axle.
- Measure the distance between the two marks on the floor. That is the reach.
Done carefully, this gets you close to the published figure. Then compare it with the reach of the bike you are considering, and note how your current stem length differs from the one the new bike comes with.
What you can adjust after buying
| Change | Effect on fit | Watch out for |
|---|---|---|
| Stem length | Moves the bars forward or back, usually by 10mm to 20mm | Trail bikes are designed around short stems, around 35mm to 50mm. Going much longer makes steering slow and puts weight over the front. |
| Handlebar rise and spacers | Raises or lowers the bars, changing effective stack and slightly shortening effective reach | Limited by the steerer tube length left above the headset |
| Handlebar width | Wider bars add leverage and stretch you slightly | Too wide for your shoulders feels awkward and catches on trees |
| Saddle position | Sets your seated pedalling position | Do not slide the saddle to fix reach. It changes your position over the pedals and affects climbing and comfort. |
These fine-tune a frame that is close to right. They cannot turn a frame that is 30mm too long or short into the right size. Our handlebar and stem sizing reference covers clamp sizes and compatibility, and our guide to setting saddle height covers the seated side of fit.
Our mountain bike size charts
- Kona MTB size chart
- Merida MTB hardtails size chart
- Polygon MTB size chart
- Cannondale Scalpel size chart
These give rider height ranges for each size. For reach and stack, check the maker's geometry table for the exact model and year, because geometry changes between generations of the same bike.
FAQ
Should I size down on a mountain bike?
Only if you are between sizes and value agility over stability, because modern mountain bikes are already much longer than older ones, and a size that once felt huge is now normal, so sizing down by habit often leaves riders cramped on climbs and nervous at speed. Sizing down does make a bike livelier in tight trails and easier to manoeuvre. If your height sits clearly in one size band, ride that size. The old advice to always size down dates from a time when frames were short and steep.
Do 27.5in and 29in wheels change the size I need?
The wheel size does not change your reach, but it does change how the bike fits around you, because a 29in wheel sits higher and can make the front end and standover taller on the smallest frames, which shorter riders notice most when stopping or getting behind the saddle. That is why many brands fit 27.5in wheels to their extra-small and small sizes, or offer both. Riders under about 165cm often find a 27.5in bike easier to handle and to get their weight over. Above that height, 29in wheels roll better over rough ground and suit most riders.
Does a dropper post affect which size I should buy?
It can, because the seat tube length and how far a post can be inserted limit how long a dropper you can fit, and a longer drop gives you more room to move your body on steep descents and technical sections, which is much of the point of having one. Measure your saddle height, then check that the frame can take a dropper with useful travel at that height. On some frames a shorter rider can only fit a short-travel dropper on the larger size, which is a reason to choose the smaller one. Frame makers list the maximum insertion depth in their geometry or manual.
Can I use a longer stem to make a small frame fit?
Only within a small range, because a mountain bike's handling is designed around a short stem, and fitting a long one to make up for a frame that is too short slows the steering and puts too much weight over the front wheel on descents. A change of 10mm to 20mm to fine-tune fit is normal. If you need to go from a 40mm stem to 80mm or more, the frame is the wrong size, and the right answer is a frame with more reach.
What is the difference between reach and top tube length?
Top tube length, often listed as effective top tube, is the horizontal distance from the top of the head tube back to the line of the seatpost, so it depends on the seat tube angle, while reach is measured forward from the bottom bracket and ignores the seat angle altogether. That makes reach the better number for comparing frames. Two bikes with the same top tube length can have very different reach if their seat angles differ, which is common between older and newer designs.
Getting your size right
If you are between sizes, or comparing bikes from different brands, a fit takes the guesswork out. See our bike fit service, or ask TBA Guide with your height, inseam and the bike you are looking at, and we will help you choose the size.
