Saddle Setback Calculator — Fore-Aft Position

Two numbers. The "seatpost line setback" is the horizontal distance from the bottom bracket to the seat-tube/seatpost line at your saddle height — geometrically SH × cos(STA), typically 190–230 mm. The "saddle nose setback" is the figure most riders mean by "setback": the horizontal distance from the BB to a vertical line through the saddle nose, which lands in the 40–80 mm road range once the seatpost offset and saddle position are accounted for.

They measure different points. The seat-tube line at saddle height sits roughly 200 mm behind the BB, but the saddle itself is ~270 mm long and extends well forward of its rail clamp, so the nose ends up only 40–80 mm behind the BB. Both are correct measurements of different reference points.

Road bikes: 40–80 mm behind the BB (UCI rules require the nose to be at least 50 mm behind the BB for mass-start racing). Time-trial / triathlon: 0–20 mm, sometimes forward of the BB. Mountain bikes: 50–70 mm. The KOPS (Knee Over Pedal Spindle) guideline is a starting point, not a rule — Sheldon Brown notes it has no biomechanical basis and is just a convenient reference.

More setback (further back) opens the hip angle and favours climbing and endurance. Less setback (further forward) rotates you over the BB for time trialling and steeper, more direct power delivery.