Stem length calculator
A stem is bolted to a steerer that leans back, so changing it moves your hands forward and up at the same time. Put your current cockpit on the left and the one you are considering on the right.
Your hands move
18 mm back · 26 mm up
Measured at the hoods, with the same frame, handlebar and saddle in both columns. A stem change moves your hands in both directions at once, which is why swapping 10 mm of length is never just 10 mm of reach.
Add your measurements and the bike, and it picks the stem for you.
Why 10 mm of stem is not 10 mm of reach
The stem rotates with the steerer, so its length runs slightly uphill on a −6° stem fitted to a 73° head tube. Ten millimetres more stem gives about 10 mm of reach and 2 mm of height. Ten millimetres of spacers is worse: you gain 10 mm of height but lose about 3 mm of reach, because the steerer leans back as it rises.
Flipping the stem is the free adjustment
Most stems are symmetrical, so a −6° stem becomes +6° when you turn it over. On a typical road bike that lifts the hands 18–20 mm and pulls them about 6 mm closer — usually more than you expect. Try it before buying anything, and remember the spacers you have already: stack can be added but never taken away below the head tube.
What counts as a normal stem
Stock road stems run 70–120 mm and ±6 to ±8 degrees; 40 mm and 140 mm exist but change the steering noticeably. If getting comfortable needs a stem outside that range, or more than about 30–40 mm of spacers, the frame size is at the edge for you — which is worth knowing before you buy rather than after.
Handling, not just fit
A shorter stem quickens the steering and shifts weight off the front wheel; a longer one calms it down and adds front-wheel grip when climbing. The difference is real but small next to frame geometry, so choose the stem that puts your hands in the right place first.