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Seat Tube Angle Calculator — Effective STA
Calculate the effective seat tube angle from saddle position.
How We Calculate This
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°.
Frequently Asked Questions
Effective STA is the angle between a line from the BB to the saddle top and the horizontal. It differs from the actual seat tube angle when the saddle is moved forward or backward on its rails.
Road bikes: 72-74 degrees. TT bikes: 75-80 degrees. Mountain bikes: 73-76 degrees. Steeper angles put you further forward over the pedals.
It affects your position over the pedals, power delivery, and muscle recruitment. Steeper angles engage quadriceps more, while slacker angles use more glutes and hamstrings.
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Last updated: June 2026
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.