Mountain bike frame size calculator
On a mountain bike, S, M or L says even less than on a road bike: two size Ms can differ by up to 60 mm of reach. What every maker measures the same way is stack and reach. Here you get the window for them, from your height, your riding style and the seat angle of your bike — for hardtail and full-suspension.
Another kind of bike? Frame size for road, gravel and trekking bikes
Without an inseam we estimate it from your height. A measured inseam makes the window more accurate: Barefoot, back against the wall, a book pulled firmly up into your crotch — then from the floor to the top of the book.
Arm length: let your arm hang loose and measure from the point of the shoulder (the bony tip at the top outside of the shoulder) to the bone at the wrist. The hand does not count — how long your fingers are does not matter for your riding position. It is easiest with a second person. Each centimetre of arm moves the reach by 6 to 7 mm.
Why the seat angle matters on a mountain bike
On a mountain bike the seat angle runs from 74 to 78° and belongs to the bike, not to you: it moves the saddle by 12 to 13 mm per degree. At 179 cm the reach window is 430 to 460 mm with 74°, 445 to 470 with 75° and 470 to 495 with 77° — from 74 to 77° there is more than one size in between. The angles of a suspension bike are in the maker's geometry chart: a full-suspension bike builds a steeper seat angle than a hardtail.
Hardtail, full-suspension and e-MTB
The calculation has no switch for suspension: when the bike sags, the main frame turns with you as a whole, and knee angle, shoulder angle and saddle height stay. On an e-MTB it is the same question — the motor changes nothing about the riding position.
Two size Ms are not the same
As an example, one bike with sizes from S to XL — not a recommendation, just a scale for how stack and reach grow with height. Another model of the same size can differ by up to 60 mm of reach.
| Size | Body height | Stack | Reach | Seat angle |
|---|---|---|---|---|
| S | 166–175 cm | 608 | 430 | 75° |
| M | 175–183 cm | 622 | 450 | 75° |
| L | 183–192 cm | 640 | 470 | 75° |
| XL | from 192 cm | 658 | 490 | 75° |
What stack and reach are
Both numbers are measured from the same point: the centre of the bottom bracket to the top of the head tube, where the stem sits. Stack is the vertical distance — it tells you how high the front of the bike is. Reach is the horizontal distance — it tells you how far ahead of you the handlebar is.
How to find the right size
- In your manufacturer's geometry chart, find the “Stack” and “Reach” columns. Every brand lists both.
- Choose the size whose reach lies in the window above. Reach belongs to your body: to torso and arms.
- Then look at the stack. If it is below the window you will sit lower than you wanted; if it is above, more upright. Spacers under the stem and a stem with a rise make up part of the difference.
- Two sizes in the running? Then riding style decides: if you want to sit lower, take the lower stack.
Why a window and not a size
A 54 is a different frame at one brand than at the next, and even within one brand it changes from model to model. Stack and reach can be compared. The reach window assumes a shoulder angle as bike fitters measure it on the bike, and a stem from 40 to 70 mm.
Stack, on the other hand, depends on riding style: how low you want to sit decides which front end suits you.
What this page does not calculate
Standover height — whether you can stand over the top tube. Top tube height is not in any geometry chart this application knows, and an estimated number would be worse than none. Check it when you try the bike: standing over the top tube, there must be clearance between the tube and your crotch.
Stack and reach by height
| Body height | Stack | Reach |
|---|---|---|
| 155 cm | 560–575 | 380–410 |
| 160 cm | 570–580 | 395–420 |
| 165 cm | 575–590 | 405–435 |
| 170 cm | 585–600 | 420–445 |
| 175 cm | 595–610 | 430–460 |
| 180 cm | 605–620 | 445–470 |
| 185 cm | 615–630 | 455–480 |
| 190 cm | 625–640 | 465–495 |
| 195 cm | 635–650 | 475–505 |
| 200 cm | 645–655 | 490–515 |
And then?
The bike fit shows how you sit on a particular frame: knee angle, saddle-to-bar drop and back angle, as a 3D model you can adjust.
To the bike fit with all measurements Compare two bikes Setting up your road bike position
This is a guide value from a calculation, not a made-to-measure suit. If you want it exact, have someone look at you on the bike. This page stores nothing — your input lives only in the address.