Barefoot, back against the wall, a book pulled firmly up into your crotch — then from the floor to the top of the book.
Let the arm hang loosely and measure from the corner of the shoulder (the bone at the top outside) to the knuckle of the wrist. It is easier with two people.
What is different for women on the bike →
measure from a photo
Shoe, cleat, pedal and foot angle move the foot on the pedal, and with it the right saddle height.
Race, endurance or gravel? At the same size the bar sits 2 to 3 cm higher on endurance and gravel bikes than on a race bike. The difference, with a picture
Stack and reach, once entered, replace the size. Triathlon bikes are not covered.
On the maker's site under “Geometry”, one column per frame size, in millimetres. You can't read them off the bike itself.
What stack and reach are: Bike explained →
BianchiBMCBullsCannondaleCanyonCubeFocusGhostGiantKTMMeridaRoseScottSimplonSpecializedStevensTrek
Older model? These collections often list past model years too: Geometry Geeks99 Spokes
The angle is printed on the side of the stem. Flipped, it raises the bar by about 2 cm. Spacers are the rings under the stem, 5 mm each.
The crank length is printed on the inside of the crank arm. Left empty, the one that suits your inseam applies.
The setback is printed on the seatpost: 0 for a straight one, otherwise usually 10 to 25 mm. Left empty, 10 applies.
The seat angle is in the geometry chart (“seat tube angle”). On a mountain bike it moves the saddle by several centimetres. Left empty, the angle of a calibrated road bike of similar size applies.