Frame size and fit
Two answers, because the industry gives two. A seat-tube number from your inseam, and the size band a manufacturer would sell you from your height. Where they disagree, that is worth knowing before you buy.
How to measure this
4 steps1Measure your height
Barefoot, heels and back flat against a wall, looking straight ahead. Mark the wall at the top of your head, then measure down to the floor.
Your measurements
Barefoot, back to a wall, a book pulled up firmly between your legs. Measure from the top of the book to the floor.
Starting fit
Centre of the bottom bracket to the top of the saddle, measured along the seat tube.
A proportional starting point. Fitters increasingly go shorter than this, especially for hip comfort and aero positions.
The frame's top tube height should not exceed this, or you cannot stand over the bike safely.
A rough starting point only. This one genuinely needs a fitter.