1 · Your body
Inseam: stand barefoot against a wall, hold a book level between your legs, measure down to the floor.
Fine tuning — saddle, shoe, pedal
The setback is printed on the seatpost — straight, or offset 15 to 25 mm to the rear; 15 is the most common. How far behind the bottom bracket the nose of the saddle ends up is worked out by the page: that also depends on the seat angle and the saddle height, which is why it is a different number on every frame. The other four fields move the foot relative to the pedal axle.
2 · Your bike
Crank length (mm) — optional
Frame colour
Steel blue
Charcoal
Black
Silver
Forest green
Copper
Teal
Sand
Jersey colour
Orange
Black
White
Blue
Green
Purple
Grey
Stack and reach are in every manufacturer's geometry table. Entering both overrides the frame size you picked.
The crank length is usually printed on the inside of the crank arm. It moves the bottom of the pedal stroke and with it the knee angle: between 165 and 172.5 mm there are about three degrees. Without it the page works with 172.5 mm — the usual length on a road bike.
The type decides where your hands are — and with that, how upright you sit: on a flat bar you grip behind the stem clamp and above it, on a drop bar 90 mm in front of it. Triathlon is deliberately missing: there the rider lies on the forearms, and that is not a different setting but a different calculation.
Where do I find stack and reach?
In your bike's geometry table: open your model on the manufacturer's site and go to the “Geometry” tab. Stack and reach are given in millimetres, one column per frame size. You cannot measure them off the bike itself — they are derived figures, not tube lengths.
Bianchi BMC Bulls Cannondale Canyon Cube Focus Ghost Giant KTM Merida Rose Scott Simplon Specialized Stevens Trek
An older model year the manufacturer no longer lists? These collections often carry old models too. They are other people's sites — this page fetches nothing from them, it only points the way:
Geometry Geeks 99 Spokes