Write down your current bike’s stack and reach, find frames whose numbers are as close to yours as possible, then use the stem and spacers to cover small remaining gaps. Do not trust the size label: Canyon notes that a medium in one brand can feel totally different in another, and even within one brand a medium aero road bike rides differently from a medium endurance bike.
What the two numbers mean
Stack is the vertical distance from the bottom bracket (the crank axle) up to the top of the head tube. Reach is the horizontal distance from the center of the bottom bracket to the head tube. A bigger stack gives a more upright position, and a longer reach stretches you out with your hands further forward.
Both are measured from the bottom bracket, so they ignore seat angle, seatpost setback and saddle position, according to BikeTips. That is why they travel between brands better than seat tube length, which says nothing about how long or tall the front end is. They are best read as a pair, in millimeters (1 mm is about 0.04 in).
Step by step
- Confirm your current bike really fits. This method copies a position, so it only helps if you are happy with the one you have. If you are not sure, start with how to fit a bike properly and set your saddle first, using saddle height guidance.
- Get your current frame’s numbers. Canyon advises jotting down the stack and reach of your current bike before ordering. Use the manufacturer’s geometry chart for your exact frame size.
- Add your cockpit. Frame stack and reach stop at the head tube. Where your hands end up also depends on stem length and angle, the spacers under the stem and the handlebar. Canyon also lists Stack+ and Reach+ figures and tells riders to check them for comfort. Because those are separate figures from the plain frame numbers, compare Stack+ with Stack+ and frame stack with frame stack, never one against the other.
- Compare like with like. Pull the chart for each candidate frame and look for the same figures. BikeTips says a frame with matching stack and reach should put you in a near-identical position whatever size label it carries.
- Cover the gap with the cockpit. See the table below.
What the stem and spacers can fix
| Frame difference | Fix | Limit |
|---|---|---|
| Reach slightly short | Longer stem | Small changes only |
| Reach slightly long | Shorter stem | Small changes only |
| Stack slightly low | Spacers under the stem or a higher-rise stem | Small changes only |
| Stack or reach far off | Choose another frame size | Cockpit parts cannot fix it |
BikeTips puts the limit bluntly: you cannot meaningfully shrink a frame that is too long or stretch one that is too short, and you can only move your effective position around the frame’s fixed numbers.
Stack changes also move your hands relative to the saddle, so after any change recheck how your back, shoulders and hands feel on a ride. Have a shop check the cockpit if you are unsure about any change to stem height.
Frames of different types
If the new bike is a different type from the old one, the same numbers may not be what you want. Canyon says race bikes use longer reach and lower stack for an aerodynamic position, while endurance bikes have shorter reach and higher stack. BikeTips adds a rule of thumb using the stack-to-reach ratio: about 1.4 is typical of an aggressive race bike, and 1.5 or higher signals a taller, more upright endurance or all-road position. Treat that as a sorting tool, not a target.
Switching from a race bike to an endurance frame, for example, you may deliberately choose a taller stack than your old bike. Switching the other way, check that you can hold the lower position comfortably; a low position suits riders who are flexible enough to hold it, which is worth testing before you commit.
When you fall between sizes
Look at which number is further off. If reach is slightly short, a slightly longer stem is the easy fix. If stack is a little low, spacers are. If both numbers sit between two sizes, compare the charts of both sizes rather than guessing, and if you can, test ride or ask a fitter. For kinds of bike you have not ridden before, see road vs hybrid vs mountain first, as the geometry targets differ by type.
Stop and see a professional fitter if a new position causes numbness, pain in the wrists, neck or knees that does not ease after a few rides, or if you are unsure the stem and spacers can be fitted safely.
Sources
Last reviewed: October 2026.