What are stack and reach?
Stand a frame at the bottom bracket and measure to the top-center of the head tube: straight up is stack, straight forward is reach. Between them they fix where your handlebars can go — which is to say, they fix the fit. They're the only two geometry numbers measured the same way by every manufacturer, which is why they cut through the size-label chaos our frame size calculator warns about: a "54" is marketing; 565 mm of stack is a fact.
What's a good stack-to-reach ratio?
Divide stack by reach and you get a one-number personality test for any frame — higher means more upright:
| Stack ÷ reach | Character | Rides like |
|---|---|---|
| under 1.4 | Aggressive race / TT | Long and low — flexibility required |
| 1.4–1.5 | Race | The classic road-race position |
| 1.5–1.6 | Endurance | Higher bars, all-day back |
| 1.6+ | Upright / comfort | Gravel adventure, hybrids, relaxed riding |
Approximate conventions — plenty of bikes straddle the lines, and a long seatpost or slammed stem moves any frame a band in practice. Rendered from the same bands the calculator uses.
Comparing two frames — the translation nobody does
Here's the part the geometry charts leave as homework. Take a rider on an endurance 56 (stack 598, reach 380 — ratio 1.57) eyeing a race 56 (stack 565, reach 395 — ratio 1.43). The race frame is 33 mm lower and 15 mm longer — sounds dramatic. But spacers don't stack straight up: they climb the steerer tube at the head-tube angle, so every 10 mm of spacer buys ~9.6 mm of height and pulls the bars ~3 mm back. Run the trigonometry and matching your old position on the race frame takes about 35 mm of spacers and a 5 mm shorter stem — the scary 15 mm of extra reach mostly evaporates. The calculator above runs this for any two frames, which is exactly the question to answer before buying.
At a typical 73° head angle: 10 mm of spacers = +9.6 mm stack, −2.9 mm reach. Steeper head tubes trade slightly more height for less reach loss; slacker ones the reverse — which is why the calculator takes the angle. Practical limits while you're moving things: most carbon steerers cap spacer height around 30–40 mm (check your fork's manual — that one's a safety limit, not a style guide), and stems swap sanely between about 60 and 130 mm before the steering feel changes character.
Across mainstream brands, frames wearing the same size label routinely differ by 20–30 mm of stack — more than the entire gap between our race and endurance archetypes above. That's why "I ride a 56" transfers between brands about as reliably as shoe sizes between countries, and why the two numbers on this page are the ones to write down and carry into every future bike search. Your saddle position, meanwhile, travels with you — set it with the saddle height calculator and it's the same on every frame.
What stack and reach don't tell you
Two numbers describe the front of the fit, not the whole bike. They're silent on wheelbase and trail (how it steers), front-center (toe overlap), seat-tube angle (where your saddle clamp ends up — which can quietly change effective reach by a centimeter), and bar shape (a long-reach handlebar eats stem length). Rule of thumb: stack and reach decide whether a frame can fit you; the rest decides how it rides once it does. A full geometry-chart comparison tool is on our roadmap for that second half.
Using this before you buy
The workflow: pull stack and reach from your current bike's geometry chart (the bike that fits — or fits almost, noting what you'd change), enter each candidate as frame B, and look at the translation. Needs under ~20 mm of spacer change and a stem between 80 and 120? The frame can reproduce your position with room to fine-tune. Needs 40 mm of spacers or a 140 mm stem? You're buying the wrong size, however right the label sounds.