Measure yourself first, then read the chart, and know which numbers predict fit
How to take inseam, torso and arm measurements at home with a hardback and a wall, then use them to read stack, reach, effective top tube and standover on a geometry chart.
One rider got tired of being handed a size down instead of a shape that fit, measured a lot of frames, rode a few, and wrote down what actually made the difference. Everything here is checkable in a shop with a tape measure.
Height alone predicts little
A hardback pulled firmly upward between the legs mimics the pressure of a saddle better than a ruler laid flat. Light pressure gives a short reading and a saddle set too low.
A shop that asks your height and hands you a frame has skipped the only step that costs nothing. Height is a poor predictor of fit because it says nothing about how the height is distributed, and two riders at five foot two can differ by three inches of inseam, which changes standover, saddle height, and the whole relationship between hips and handlebar. The measurements that matter take about fifteen minutes, a hardback book, a wall, and a helper who can hold a pencil steady. Do them before you look at a single frame.
The inseam measurement, done the way it is meant to be done
Stand barefoot with your back against a flat wall and your heels about six inches apart, roughly the width they would sit at on the pedals. Take a hardback book, pull it up between your legs with real pressure, firm enough to mimic a saddle rather than merely resting there, and keep the spine level and square to the wall. Have someone mark the wall at the top edge of the spine, then measure from the floor to the mark. Do it three times. If the three readings spread more than about a centimeter, you are varying the pressure, so repeat until they agree.
That number drives more decisions than any other. It sets your saddle height, it tells you whether standover on a given frame leaves usable clearance, and on bikes with sloping top tubes it determines how much seatpost you will have showing, which in turn affects whether the post is long enough and whether the bar can ever come up to meet you. Record it in centimeters, because geometry charts are published in millimeters and converting once is easier than converting repeatedly.
Torso and arm, and why they change the answer
For torso, sit upright on a hard chair against the wall and measure from the seat surface to the notch at the top of your breastbone, keeping the tape vertical rather than following the curve of your chest. For arm, hang the arm relaxed at your side and measure from the bony point at the tip of the shoulder to the crease of the wrist. Neither number maps to a frame dimension directly, and anyone who tells you otherwise is selling a formula. What they do is settle the question the inseam cannot answer: whether a frame that fits your legs will also let you reach the bar without locking your elbows or hunching your shoulders.
Shorter riders are frequently long in the torso relative to their legs, or short in both, and the two patterns want different bikes. The pattern shows up when you compare torso plus arm against inseam. Longer combined reach relative to inseam means the standard-length stem on a small frame will probably be fine. Shorter means the frame will feel stretched even at the correct saddle height, and the fix is a shorter stem, a narrower and shallower bar, or a frame with less reach to begin with.
Reading the chart, and which columns are doing the work
Stack is the vertical distance from the bottom bracket to the top of the head tube, and reach is the horizontal distance between the same two points. Together they describe where the front of the bike sits relative to where you pedal, independent of seat tube angle and seatpost setback, which is why they predict fit better than anything else on the chart. Effective top tube is measured to a horizontal line at the top of the head tube and is useful for comparing bikes of similar seat angle, but it mixes seat position and front-end position into one figure, so it flatters a slack frame and penalizes a steep one. The National Institute of Standards and Technology is responsible for the measurement infrastructure that makes such published dimensions comparable across manufacturers, and the charts genuinely are consistent about how stack and reach are defined.
Standover deserves attention but not deference. It is a floor, not a target: enough clearance to stop safely is enough, and a frame that clears by four inches is not thereby a better fit than one clearing by two. Careful readers check standover last, after stack and reach have narrowed the list, and then confirm the seatpost has enough insertion depth at their saddle height and that the head tube is tall enough to put the bar where the torso and arm numbers say it should go.
Write your four numbers on a card and carry it. A frame that matches them will feel unremarkable on the first ride, which is exactly the result you are looking for.