The formula
road frame = inseam × 0.665 · saddle = inseam × 0.883
Where it comes from
Two people of the same height can have very different inseams, and it is the inseam that decides whether you reach the pedals and whether you can put a foot down with the bike between your legs. Each factor comes from the typical geometry of the discipline: mountain frames are measured in inches and built more compact for room to move.
How to work it out by hand
- Measure your inseam barefoot, back to the wall, with a book acting as the saddle
- Multiply by 0.665 for road, 0.226 for mountain or 0.685 for hybrid
- For the saddle, multiply the inseam by 0.883 and measure from the bottom bracket
What is worth knowing
How to measure the inseam properly: barefoot, back against a wall, with a book between your legs pressed upwards the way a saddle would, measured from the floor to the top edge of the book. Saddle height comes from Lemond's formula, measured from the bottom bracket to the centre of the saddle along the line of the seat tube. If you are between two sizes, the smaller is usually the better bet: a longer stem fixes a small frame, but a large frame cannot be fixed at all.