Stack and Reach Explained: How to Size a Bike Properly

Frame sizes lie. A 54 from one brand fits nothing like a 54 from another, because the label refers to a seat tube length that no longer describes how a bike fits. Two numbers do describe it: stack and reach. This guide explains what they are, how to read a geometry chart, and how to use them to choose a size with confidence.

This is part of IKUISTA Tech Notes, our plain-language guides to the engineering behind the bike.

What stack and reach measure

Both are measured from the same two points: the centre of the bottom bracket, and the top centre of the head tube.

Stack is the vertical distance between them. It controls how high your handlebars sit.

Reach is the horizontal distance between them. It controls how far forward you are stretched.

Together they place the front of the bike relative to your feet. That is most of what fit is. The idea was pushed into mainstream use by Cervélo, and it caught on because it removed the guesswork from comparing frames.

The reason they work is that they are independent of tube angles and lengths. Seat tube length and top tube length shift with design decisions. Stack and reach do not. So a size 54 road frame and a size Medium gravel frame can be compared directly.

Reading a geometry chart

Look at stack first, then reach, then divide one by the other.

As a broad guide for road frames, a stack below about 530mm is aggressive race territory, 530 to 555mm is performance all-rounder, and above 555mm is endurance. For reach, above about 400mm is aggressive, 385 to 400mm is all-rounder, and below 385mm is endurance.

Then divide stack by reach. That ratio is a single number describing how upright or aggressive the position is. A low ratio, around 1.40 or below, is aggressive and aerodynamic, which asks for flexibility and core strength. Around 1.40 to 1.50 is the performance all-rounder zone where most fast, comfortable bikes sit. Around 1.50 to 1.60 is endurance, more upright and easier over long days. Above about 1.60 is relaxed touring and gravel territory, favouring comfort and visibility over aerodynamics.

An example makes it concrete. A frame with 548mm stack and 388mm reach gives a ratio of 1.41. That is a performance all-rounder: quick enough for hard group rides, civil enough for long ones.

Getting to your own numbers

A useful starting point uses your inside leg measurement. Stand barefoot with your back against a wall, press a hardback book up firmly into your crotch, and measure from the top of the book to the floor. That is your inside leg.

A long-standing rough estimate multiplies inside leg by 0.67 to approximate a target stack. Then divide that stack by the ratio you want, based on the position you are after. For someone with an 85cm inside leg wanting a sporty 1.48 ratio, that gives roughly 570mm stack and 385mm reach.

Treat these as a filter, not an answer. They narrow the field to two or three candidate sizes. Your flexibility, injury history, and riding style decide the rest.

How much you can adjust

A frame does not have to match your targets exactly, because the cockpit gives you room.

Stack can be adjusted upward with headset spacers, typically over a range of 0 to about 25mm, and further with a rising stem. If a frame's stack is within about 15mm of your target, it is workable.

Reach can be adjusted with stem length. Stems from around 80 to 120mm give roughly 20mm of effective adjustment either way.

Note the asymmetry. It is easy to raise a bar, and harder to lower one below the frame's stack. So if you fall between two sizes and want a lower, sportier position, the smaller size gives you more room to work with. If you want comfort, the larger size gets you there without a tower of spacers.

The limits of the two numbers

Stack and reach do not describe everything. They say nothing about how a bike handles. Head tube angle, trail, wheelbase, and chainstay length decide stability and steering feel, and two frames with identical stack and reach can ride very differently.

They also describe the frame, not the bike as delivered. Stem length, bar shape, spacer stack, and saddle position all shift your final position. Some brands publish a stack-plus and reach-plus figure measured to the handlebar tops for exactly this reason.

And no chart replaces a professional bike fit. A fitter accounts for flexibility, old injuries, and how you actually pedal. Use geometry to shortlist. Use a fit to finish.

The IKUISTA approach is to publish clear geometry and help you interpret it before you buy, rather than hiding behind size labels.

Your turn

What stack and reach are you riding now, and does the position suit you? Post your numbers in the comments. If you are between sizes and want help reading a geometry chart against your measurements, get in touch and we will work through it with you.