F.I.S.T STEM Calculator

All values are in millimeters! Except for angles, which are in degrees.

Head Angle
Stem Length
Stem Angle

Plug in actual stem pitch. Write 0 [the number zero] for an extension perpendicular to the steerer; -6 for 6° down from perpendicular, 6 for 6° up; -17° for a "7-shaped" stem; and so forth.
Stem Clamp Height
Spacer Height
Headset Topcap
Agree to Terms

The calculator above is designed to help fitters trained in the F.I.S.T. tri bike fit protocol to generate geometries for custom triathlon bikes. Note that the inputs required to use this calculator include "stack" and "reach." These values are described at length in our tri bike fit articles on Slowtwitch.com.

Even if the fit is performed correctly, pitfalls loom. You must have the tools to accurately find the stack and reach of the bike that fits under the rider. The fitter is required to account for the stem length, stem pitch, height of the stem clamp, headset upper stack, spacers under the stem if any, and of course the choice of aerobar. He then must have an accurate way of generating the stack and reach measures, using either a fit bike (best is the bike made by Exit Cycling) that accurately generates these values (taking into consideration negative reach if applicable), or he must use an external tool, such as Serotta's X/Y tool.


This calculator consists of a set of trigonometric and geometric equations. It solves math equations. That's all it does. There are no warranties expressed or implied that these geometries cause a bike to be safe, or comfortable, or powerful, or aerodynamic, or even be ridden; or indeed can even be built. Should you build or cause to build a bike based on geometries calculated by this generator, you do so at your own risk. You agree to hold harmless Slowtwitch.com, its owners, employees and contractors against all claims arising out of your use of this calculator. You agree to assume all risk and liability associated with using this calculator. You agree that the final determination of an appropriate geometry rests with you and/or any customer for whom you may be building this bike, or causing it to be built, and with the company building the bike.