1 · Your body
Inseam: Barefoot, back against the wall, a book pulled firmly up into your crotch — then from the floor to the top of the book.
Arm span: stand with your back to a wall, arms stretched out level to the sides. Stick a piece of tape to the wall at each fingertip, then measure the distance between the tapes. You can do it alone — and the length of your arms changes your back angle more than you would think.
Fine tuning — saddle, shoe, pedal
Shoe, cleat, pedal and foot angle move the foot relative to the pedal axle — and with it the saddle height that goes with a knee angle.
2 · Your bike
Stack and reach are in every manufacturer's geometry table. Entering both overrides the frame size you picked.
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
Fine tuning — frame, seatpost, cockpit, colour
The number is usually printed on the side of the stem, often as "±6°" or "17°". Whether it raises or lowers the bar depends only on how it is mounted: flipped, the same stem raises the bar by about two centimetres. Spacers are the rings under the stem, 5 mm each.
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 crank follows your inseam, in the sizes you can actually buy.
The setback is printed on the seatpost — 0 for a straight one, otherwise usually offset 10 to 25 mm to the rear; without an entry the page uses 10. 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 seat tube angle is in the geometry table. It decides how much of the saddle height acts horizontally rather than vertically, and with it the setback to the bottom bracket — on a mountain bike that amounts to several centimetres. Without it, the page uses the angle of the nearest calibrated frame size, and that one is measured on a road bike.