Bike Geometry Comparison & Fit Calculator
Find your fit from body measurements
What these numbers mean
Reach and stack (the two that matter most)
Reach is the horizontal distance from the bottom bracket to the top of the head tube; stack is the vertical distance. Together they define the cockpit far better than the old frame-size label.
Reach-to-stack ratio
Reach divided by stack gives a quick read on posture. Under 1.4 is upright/endurance, 1.4-1.5 balanced/trail, above 1.5 aggressive/race.
Head tube angle
Slacker (e.g. 64 deg) is more stable descending; steeper (e.g. 71 deg) steers quicker. A 1 deg change is noticeable.
Seat tube angle
Steeper effective seat angles push your hips over the bottom bracket, better for climbing and modern long-reach bikes.
Wheelbase and chainstay
Longer wheelbase is more stable; shorter chainstays are more playful. They trade off for handling feel.
What a bike geometry comparison actually tells you
Two bikes can share a category, a wheel size and a travel figure and still fit and handle nothing alike. The numbers that decide it are reach, stack, head angle, seat tube angle, chainstay length and wheelbase. When you compare bike geometry side by side, you are really answering three questions: will it fit me, will it suit where I ride, and how different is it from the bike I already own.
The comparison tool above draws on a database of models built from the geometry guides on this site, so you can put any two of them next to each other at any frame size.
Reach and stack decide whether it fits
Reach is the horizontal distance from the bottom bracket to the top of the head tube, and it governs how roomy the bike feels when you are standing on the pedals. Stack is the vertical version of the same measurement and governs how high the bars sit.
Brands do not agree on what a size label means. The Specialized Epic 8 runs from 420mm of reach in small to 500mm in XL, an 80mm spread across four sizes, which works out to roughly 25mm per step. That is larger than the gap between some brands’ medium and large. A medium from one manufacturer can measure closer to another’s large, which is exactly why comparing the size label is useless and comparing the numbers is not.
Head angle and wheelbase decide where it wants to go
A slacker head angle pushes the front wheel further out, which adds stability at speed and on steep ground, and costs you some sharpness in tight, slow corners. Wheelbase compounds the same effect. The Pivot Firebird spans 1179mm to 1299mm of wheelbase across its size range, and that 120mm difference is felt far more than the head angle on its own.
Seat tube angle is the number most people skip
It sets where you sit relative to the bottom bracket when you are climbing. Steeper puts your hips forward and over the pedals, which keeps the front wheel down on steep climbs. The Epic 8 sits at 76 degrees in small, the Firebird at 77 degrees. On paper that is one degree. Over a long seated climb it changes where your weight lands.
Chainstay length decides how it corners
Shorter chainstays make a bike easier to lift and flick. Longer ones keep weight balanced between the wheels when things get fast. The Epic 8 uses 435mm chainstays across the range, which is on the shorter side and part of why it handles as nimbly as it does.
How to compare bike geometry without guessing
- Start from the bike you already ride. Absolute numbers mean little until you have a reference point you have actually felt. Put your current bike in one side of the tool and the candidate in the other.
- Compare at the size you would really buy, not the size label you assume. Size up and size down as well, since the answer often sits one step away from where you expected.
- Read the differences, not the absolutes. 10mm of reach is noticeable on the first ride. 2mm is not. One degree of head angle is worth paying attention to. A tenth of a degree is noise.
- Check that the differences agree with each other. A longer reach paired with a slacker head angle and a longer wheelbase all point the same way. When numbers pull in opposite directions, the bike is a compromise and it is worth knowing which way it leans.
Which numbers matter most for how you ride
- Cross-country: seat tube angle and reach, because you spend the day seated and pedalling.
- Trail: the balance of reach against chainstay length, since you want it to climb and still turn.
- All mountain and enduro: head angle and wheelbase, because descending speed is the point.
- Downhill: wheelbase above everything else.
- eMTB: chainstay length, since the motor and battery move the weight balance rearward before you even sit on it.
Frequently confused measurements
Effective top tube and reach are not the same thing, and only reach is comparable across bikes with different seat tube angles. Standover height tells you almost nothing on a modern full suspension frame. Bottom bracket drop is more useful than bottom bracket height when comparing bikes on different wheel sizes.