Start with a bicycle that fits you, not a universal conversion chart. A 54 cm frame from one brand may overlap a 52 or 56 elsewhere, and two Mediums from the same company can differ because they were designed for different riding. Reach and stack give you a common frame reference, but they don’t include the stem, handlebar, spacers, saddle, or seatpost. Those parts determine where your hands and pelvis actually sit.
The useful question is not “What does Medium convert to?” It is: Can this candidate place my contact points in a suitable range, provide the clearance I need, and deliver the kind of chassis I want without an extreme setup?
Choose a trustworthy reference bicycle
Use a bike on which you can ride comfortably and control the bike for the intended duration and terrain. It should be similar enough to the candidate that the comparison has meaning. A road race position doesn’t transfer directly to an upright commuter, and a trail mountain bike’s grip position isn’t equivalent to the hood position on a gravel bike.
Record the exact brand, model, model year, marked size, wheel setup, and adjustable geometry position. Then measure the bike as it is actually ridden:
- Saddle height from bottom-bracket center to one repeatable saddle-top point
- Horizontal saddle setback from the bottom bracket to that same point
- Headset cover and spacer stack above the frame
- Stem length, angle, and orientation
- Handlebar model or its reach, rise/drop, sweep, width, and rotation
- A repeatable hand point, such as the road-hood cradle or a marked spot on a flat-bar grip
- Crank length and pedal or cleat setup when they differ materially
Photograph the setup and write down the saddle and stem markings. “It has a few spacers” won’t help when the bicycle is no longer beside you.
If your present bike causes pain or requires a clearly improvised position, don’t copy it blindly. Use it as evidence about what has and hasn’t worked, then involve a qualified fitter when the decision is expensive or symptoms persist.
Confirm that you have comparable geometry rows
Find the manufacturer charts for the exact models, then check the diagram and notes. Use How to Read a Bicycle Geometry Chart if a field or reference line is unclear.
Before subtracting any numbers, make sure both rows describe:
- The intended frame size and market version
- The listed fork travel or axle-to-crown configuration
- The correct front and rear wheel sizes
- The same kind of static or sagged condition
- A known flip-chip, headset, dropout, or suspension position
For an adjustable mountain bike, write the setting beside every row. For a road or gravel bike with an integrated cockpit, collect the cockpit dimensions and available replacement sizes at the same time. A promising frame comparison can fail if the proprietary front end cannot reach the required position.
Compare frame reach and stack first
Reach and stack locate the top of the head tube relative to the bottom bracket. Calculate the candidate’s difference from the reference:
candidate value − reference value = frame-coordinate change
If the reference has 390 mm reach and 600 mm stack while the candidate has 397 mm reach and 590 mm stack, the candidate’s head-tube reference is 7 mm farther forward and 10 mm lower. That is useful, but it is not yet a hand-position difference.
Spacers rise along the steerer axis and also move the stem slightly rearward. Stem length and angle change both horizontal and vertical coordinates. Drop handlebars add bar reach before the hoods; swept flat bars can bring the grips behind the stem clamp. Compare the complete cockpit before deciding that a 7 mm frame difference is large or small.
Small coordinate differences can often be managed with ordinary approved parts. Large corrections, or a setup that requires the saddle, spacers, or stem to sit at a limit, suggest the frame isn’t the clean match. Check the bicycle and component manuals for spacer, steerer, stem, and installation limits rather than relying on a generic maximum.
Rebuild the hand position, not only the stem length
Two bikes with the same 80 mm stem may put the hands in different places. Head angle changes the direction of the steerer and upper stack. Stem angle changes the clamp location. Bars differ in road reach and drop, mountain-bike backsweep and upsweep, and overall rise. Hood placement and bar rotation add another layer.
Use the Velopedia Bike Fit calculator to convert frame reach and stack, upper stack, stem geometry, and a chosen hand-reference offset into bottom-bracket-relative coordinates. Run the reference bicycle first, then the candidate with parts that are actually available and approved for it.
The goal isn’t necessarily to duplicate every millimeter. A new bike may serve a different riding purpose. The worksheet shows whether a change comes from the frame or the cockpit and whether the proposed position is physically achievable.
On an integrated road cockpit, verify the available width, effective stem length, spacer parts, hose-routing requirements, and whether changes require brake bleeding. On a mountain bike, avoid using an abnormally long stem to disguise a short frame if it also changes steering leverage and the rider’s standing position in a way you don’t want.
Compare the seated position separately
Frame reach excludes the seat tube, so equal reach does not mean equal seated length. Read effective top tube and effective seat angle together, then place the saddle at the rider’s actual height.
A steeper effective seat angle moves the theoretical seat axis forward and can shorten effective top tube even when frame reach is longer. The effect can increase with saddle height. Curved or offset seat tubes make the published effective angle dependent on the manufacturer’s reference height, so an actual saddle-setback measurement is more reliable than angle alone.
Set saddle height for the rider and pedaling system, then see whether the required setback fits within the seatpost and saddle-rail limits. Don’t shove the saddle to the end of its rails merely to repair handlebar reach; saddle position primarily serves pedaling and support. After the saddle is established, evaluate the saddle-to-hand relationship.
Check seat-tube length, minimum insertion, maximum extension, post offset, and available dropper insertion. A frame can match reach and stack yet fail because the saddle cannot go low enough, the post cannot be inserted far enough, or the desired dropper travel won’t fit.
Keep clearance checks in the decision
Standover should be read at the manufacturer’s stated point and in the published tire and suspension configuration. It is one useful clearance check, not a size conversion. Confirm that the rider can mount, stop, and move around the intended bicycle safely.
Also check toe overlap where relevant, heel or calf clearance, knee clearance at the bar, and tire or fender space. Smaller road and gravel frames can place a large front wheel close to the rider’s shoe. Crank length, shoe size, cleat position, fork geometry, tire volume, and fenders all contribute, so front-center alone cannot clear or condemn the setup.
Don’t confuse fit differences with handling differences
Once the contact-point range works, compare the chassis. Head angle, fork offset, wheel radius, trail, wheelbase, front center, rear center, and bottom-bracket position help explain why two equally usable sizes may not ride alike.
A longer candidate may provide more room between the wheels and feel calmer in some conditions. A shorter one may be easier to reposition in tight terrain. Neither is automatically the correct size. Geometry, tires, suspension, bar width, weight distribution, and riding technique interact, and a size change can alter both fit and the designer’s intended handling balance.
This matters when choosing between adjacent mountain-bike sizes. Some modern size systems deliberately allow rider-height overlap and use reach and front center to offer different handling choices. That doesn’t make standover, seated position, dropper insertion, or control reach irrelevant.
For road and gravel bikes, a taller stack may reduce the amount of upper stack needed, while a longer wheelbase may reflect endurance or rough-road priorities rather than simply a larger size. Compare like use cases before treating geometry as a race from shortest to longest.
Use a comparison worksheet
Keep the evidence in separate columns so an attractive size label doesn’t control the choice.
| Field | Reference bike | Candidate bike | What the difference changes |
|---|---|---|---|
| Frame reach | 390 mm | 397 mm | Candidate head-tube point is 7 mm farther forward |
| Frame stack | 600 mm | 590 mm | Candidate starts 10 mm lower |
| Effective top tube | 570 mm | 565 mm | Read with seat angle; not proof the candidate is shorter at the hands |
| Effective seat angle | 73.5° | 74.5° | Candidate’s theoretical saddle axis is farther forward |
| Stem and upper stack | 90 mm / 25 mm | Proposed 80 mm / 35 mm | Must be resolved with head angle, bar, and manufacturer limits |
| Wheelbase | 1,030 mm | 1,050 mm | Chassis is longer; does not add 20 mm directly to cockpit length |
In this example, the shorter ETT does not cancel the longer reach. The steeper seat angle changes the seat-axis intersection used to calculate ETT. The proposed cockpit may bring the hand point close to the reference, but it must be calculated and built with permitted parts. The longer wheelbase remains a handling and packaging difference even if the contact points match.
Decide between sizes by achievable setup
For each candidate size, write the least-compromised build that reaches the intended saddle and hand positions. Eliminate a size if it requires any of the following:
- Seatpost or saddle rails outside their marked range
- More steerer or spacer height than the frame, fork, headset, or stem permits
- A proprietary cockpit configuration that doesn’t exist
- Inadequate standover, dropper insertion, or safe control reach
- A component workaround that creates handling you already know you don’t want
If both sizes remain feasible, the decision can legitimately turn on riding style, chassis length, front-end height, dropper space, luggage needs, or component availability. That is a better decision than automatically sizing down for “agility” or up for “stability.”
Use Velopedia as the comparison record
Locate the exact model-year records in the Velopedia Bicycle Archive and place the relevant sizes in the Bike Compare tool. Check archived fork, wheel, stem, handlebar, crank, and seatpost specifications alongside the geometry. Use the comparison as a research baseline, then verify the current bicycle physically because components and adjustable settings may have changed.
An Archive record cannot diagnose fit or promise that a size will work. Historical charts may omit a dimension, use a manufacturer-specific convention, or describe one original configuration. A blank field should remain unknown until another trustworthy source or direct measurement establishes it.
Bring the completed worksheet to a test ride. Set saddle height and basic controls before judging the bike, ride in the positions the bike is meant to support, and note pressure at the hands, control reach, steering space, standing room, and whether the position remains usable under load. If symptoms, unusual proportions, injury history, or an integrated front end make the choice uncertain, a professional fit before purchase is cheaper than rebuilding the wrong size.
Sources and technical review
Author: Jeff South
Technical review date: August 23, 2026
- Velopedia: Bike Fit Calculator — bottom-bracket-relative saddle and hand coordinates and their stated limits
- Velopedia: Bike Geometry Calculator — consistent frame and chassis comparison inputs
- Trek: What Size Mountain Bike Do I Need? — cross-brand use of reach and effective top tube and size-overlap context
- Canyon: Road Bike Sizing Guide — manufacturer guidance on comparing stack and reach across models and sizes
- Canyon: Road Bike Geometry Explained — distinction between frame coordinates and hand-position coordinates
- Specialized: Stumpjumper 15 Geometry App — example of overlapping size choice and configuration-dependent geometry