F.I.S.T Custom Bike Calculator

All values are in millimeters!

Chain Stay

(Note that some custom tri bike makers have a standard chain stay length, because these builders fair the rear wheel with the trailing edge of the seat tube. Find your chosen bike maker's chain stay value before you send him calcs generated here.)
Seat Angle
Head Angle
Fork Offset
Fork Length
HS Lower Stack

(1.5mm is a typical value for integrated headsets; you'll want to change this if your custom builder builds with external headsets, which is frequently the case. The oft-used Chris King headset has a lower stack of 13.7mm.)
BB Drop
Rear Wheel
Wheel Size
700C (Note: Changing this will reset certain values)
Standard (Note: Changing this will reset certain values)
Long Legged
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.

The F.I.S.T. fitting protocol is a two-step process: First, the fitter finds the rider's "fit coordinates," aka his "points in space." The second part of the protocol focuses on finding the right production bike to match those fit coordinates or, in this case, generating a custom geometry that matches the fit coordinates. But, know this: It is remarkably easy to calculate a "bad" custom geometry for a rider. The culprit is most often a badly executed "Part I" of this protocol, that is, the fitter executed a poor fit session. If this happens, the calculator below will faithfully generate the geometry for a bad bike to perfectly match the bad fit. Forewarned is forearmed.

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.

Anyone who uses this calculator can change any of the defaults below. Toggling from 700c to 650c, or from standard to long-legged morphology, will change the defaults, but you can still override our defaults and insert your own values. There are explanatory notes below, and you are encouraged to heed them.


This calculator "refuses" to return geometry for bikes with front/center values less than 560mm on a 700c bike and 525mm on a 650c bike. This, because of shoe overlap. Some bike makers think these minimum values should be even higher, by 5mm or 10mm. If the calculator displays this error message, you'll need to add reach; or you'll need to shallow the head angle and add fork offset; or both.

All bike makers, custom makers included, feature a minimum head tube length for the bikes they can build. Find this value out, from your chosen custom builder(s), because this calculator doesn't know what those minimum values are for each builder. Keep in mind that if your custom builder builds with external headsets, you must change the default lower headset stack in the calculator, and you must account for the potentially significant upper stack.

Morphologies: The calculator generates a geometry for long-legged, short-torsoed riders by employing two tactical changes, designed to grant the rider a lower center of mass, and a longer base of support for his otherwise high center of mass. When you select "Long Legged" the calculator drops the bottom bracket 10 extra millimeters below the bike's centerline; and it also shallows the head angle one degree, and adds 6mm of fork offset in order to keep the trail a constant value (our default settings generate a trail of about 61mm, more or less). Of course, you can override these defaults, and if your rider wishes (let us say) a higher bottom bracket notwithstanding his high center of mass, so be it. Just change the default.

Yes, lengthening the front/center of the bike while keeping the cockpit distance and trail constant can be achieved through shallowing the head angle while adding to the fork offset. The calculator will employ this tactic when you choose the long legged morphology option. This may result in a fork with an offset greater than 45mm. Not many fork manufacturers make forks with offsets greater than 45mm, so you'll want to check availability before you build or cause to have built a bike requiring this fork offset.


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.