Seat Tube Angle Calculator — Effective STA

Effective Seat Tube Angle

Effective STA = acos(Saddle Setback ÷ Saddle Height)

The saddle setback and saddle height form a right triangle with the bottom bracket. Saddle height is the straight-line distance from the BB centre to the saddle (the hypotenuse), and saddle setback is the horizontal distance the saddle sits behind the BB. Because setback = saddle height × cos(STA), the effective angle is acos(setback ÷ saddle height).

A greater setback means a slacker (smaller) effective angle; sliding the saddle forward steepens it. As a rule of thumb, roughly 1 cm of setback change equals about 1° of seat tube angle. Road bikes typically land at 72–74°, time-trial bikes at 75–80°.