How to Calculate Stack and Reach

If you don’t have access to the manufacturer’s geometry chart, you can calculate stack and reach using basic trigonometry if you know certain frame dimensions: the top tube length, head tube angle, seat tube angle, and the front-center distance. However, the most reliable way to calculate it physically is using the “Two-Wall Method.” Lean the bike against a wall with the rear wheel touching a perpendicular wall. Measure the distance from the rear wall to the center of the BB (X1) and then to the center of the head tube (X2). The Reach is simply X2 minus X1.

For Stack, measure the height from the floor to the center of the head tube (Y2) and from the floor to the center of the BB (Y1). The Stack is Y2 minus Y1. For those using software, CAD programs or online geometry comparators can calculate these values by inputting tube lengths and angles. Understanding these calculations is essential for “geometry nerds” and bike builders who want to ensure that a custom frame will fit perfectly before the first tube is even cut, preventing costly ergonomic mistakes.