What Size Bike Do I Need? Frame Sizing Explained

Frame size is the one thing you cannot adjust later, so it is worth getting right before you buy. Height charts give a starting point, but the numbers that actually decide fit are stack and reach, and your own proportions matter more than a single height figure.

In short

Height charts are only a starting point: two riders of the same height can need different frames because leg and torso length vary. Measure your inseam as well as your height, then compare stack and reach between brands, because the same size label differs per brand. Between two sizes, most riders are better off with the smaller frame.

Bike size calculator: measure your inseam with your camera

Stand in front of your camera for five seconds. The tool finds your crotch in your body outline and turns it into your inseam, then into a frame size range. Free, no account.

  1. Barefoot, in shorts or tight trousers, hair flat.
  2. Phone upright at hip height, 2–3 m away, your whole body in view.
  3. Face the camera, feet about 20 cm apart, arms slightly away from your body, and hold still.

How to measure your inseam

A 37-second video. You need a hardcover book and a tape measure.

  1. Stand barefoot with your back and heels against a wall.
  2. Put the book between your legs, spine up, snug like a saddle, level and with its back edge against the wall.
  3. Measure from the floor to the top of the book, keeping the tape vertical. A helper makes this easier.
  4. Enter the number in centimetres.

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Rather than guessing, measure your actual position with your own camera.

Measure my fit on this bike

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Why height charts are only a starting point

Most size charts map your height to a frame size. That works as a rough filter, but two riders of the same height can need different frames, because leg length and torso length vary independently. A rider with long legs and a short torso needs a very different frame from someone the same height with the opposite build.

Measure your inseam as well as your height (barefoot, back to a wall, a book pulled firmly up between your legs). Long legs relative to your height usually means you sit at the smaller end of the range with more seatpost showing; a long torso pushes you the other way.

Stack and reach: the numbers that matter

Modern geometry tables list stack (how high the head tube sits above the bottom bracket) and reach (how far forward it sits). These describe the frame independently of seat-tube length, which is why they are the useful comparison between brands.

Comparing stack and reach is how you tell whether a size L from one brand is the same as a size L from another. Very often it is not.

Wondering where your own position sits? You can measure it in twelve seconds.

Measure my fit on this bike →

When you are between sizes

This is the most common dilemma, and the honest answer for most riders is: choose the smaller frame. A smaller frame with a longer stem and a raised bar can be adjusted into a comfortable position. A frame that is too big cannot be shortened, you run out of adjustment and end up overstretched.

The exception is if you are experienced, flexible and ride aggressively, where the larger size may suit better.

What you can adjust afterwards, and what you cannot

Adjustable: saddle height, saddle fore/aft, stem length and angle, bar height (within the steerer available), bar width, crank length (if you replace them).

Not adjustable: the frame's stack and reach, standover height, wheel size. That is exactly why sizing deserves care.

A quick sanity check when trying a bike

Standing over the frame, you want clearance between you and the top tube. Sitting on the saddle at roughly the right height, you should be able to reach the bars with a slight bend in your elbows, not locked out, and without rounding your lower back sharply to get there.

After you buy

Frame size gets you into the right ballpark; the fit happens afterwards through saddle height, fore/aft and the front end. A BikeFit/LAB analysis measures your knee extension, hip angle, torso lean and reach on the bike you actually own, so you know whether your setup sits in a sensible range or whether the stem needs to change.

FAQ

What size bike do I need for my height?

Height charts give a starting range, but stack and reach describe the frame more accurately. Measure your inseam as well: long legs relative to your height usually means going smaller with more seatpost showing.

Should I size up or down if I am between sizes?

For most riders, size down. A smaller frame can be lengthened with a longer stem and raised bars, whereas a frame that is too long cannot be shortened enough and leaves you overstretched.

What is stack and reach on a bike?

Stack is how high the head tube sits above the bottom bracket, reach is how far forward. Together they describe the frame's fit independently of seat-tube length, which makes them the reliable way to compare sizes between brands.

General position guidance, not medical advice. Individual anatomy varies; consult a physician or professional fitter for persistent pain. How BikeFit/LAB measures, and where its reference ranges come from · How accurate is an AI bike fit?

Rather than guessing, measure your actual position with your own camera.

Measure my fit on this bike

€5 for one fit, or 10 fits for €25. See pricing →