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The Fe Score: what goes into the number

By Chris Ayers 6 min read

Every morning Ferrabelle shows one number from 0 to 100, called the Fe Score. It answers one question: push, or rest? I built it, so I can tell you exactly what goes into it. I can also tell you the part I am least sure about. Both halves matter, and the second one is the reason I wrote this.

The five inputs and their weights

The score is a weighted average of five things.

Then one small adjustment. If you already logged a hard session today, or heavy cardio in the last day or two, the score drops a few points. You have spent some of the day’s headroom. That nudge is applied once, never twice.

The result sorts into four bands. 75 and up is Go: “Good day to push, if you feel ready”. 60 to 74 is Moderate: “Solid, steady day”. 45 to 59 is Easy: “Keep it easy today”. Below 45 is Rest: “Prioritize rest”.

It works with whatever you log

You will not have all five inputs every day. Most people do not. When an input is missing, the engine drops it and reweights the rest, so a morning with only a cycle phase and a sleep rating still produces a sensible number. If you log nothing but your period dates, the score is driven by phase alone: about 85 on a follicular day, about 50 on a luteal day. If there is nothing at all to go on, it sits at a neutral 60. The more you log, the smaller the share phase holds. That is by design, and as you will see, it is also the honest way round.

It shows its math

I work with data for a living, and the thing I dislike most about readiness scores is that they are black boxes. A number appears, you are told to trust it, and when it feels wrong there is nothing to check.

Ferrabelle shows every factor with its weight, today’s value and whether it is pulling the score up or down. “Sleep quality, 22%, 3/5 last night.” “Recovery (RHR + HRV), 15%, RHR 54 vs your baseline.” “Fueling, 10%, not logged.” If the number looks wrong, you can see why in ten seconds, and you can decide the app is wrong. A readiness score in a menstrual cycle app is not worth much if you have to take it on faith.

The catch, stated plainly

Here is the part I owe you. Cycle phase carries the biggest single weight in the score, 35%. And the research on how much cycle phase affects strength says the effect is trivial to small.

A 2020 review in Sports Medicine pooled 51 of 78 studies and found performance “might be trivially reduced during the early follicular phase”, with the quality of evidence rated low. The authors said general guidelines “cannot be formed” and recommended “a personalised approach … based on each individual’s response”.

A 2026 study in the same journal followed 28 resistance-trained women through about eight weeks of supervised bench press and trap bar deadlift, with phases confirmed by ovulation tests and blood hormones. Bar speed differed from expected by about 0.01 to 0.02 meters per second across phases, which is tiny. What did predict training performance was motivation to train. A readiness rating did not.

So a study designed to catch phase effects in lifters found very little, and found that a readiness rating did not predict how the lifting went. I built a readiness score with phase as its largest input.

Why keep it? Two reasons, and neither is a defense of the 35%.

First, the score is not only about strength. The year-long FENDURA study of 58 endurance athletes found that symptom severity was linked to lower sleep quality and lower readiness to train, and that symptoms were worst on bleeding days. Phase tells you something about how a day may feel, even if it tells you little about what the bar will do.

Second, the score bends toward your own logs. Every input you add shrinks the share phase holds. For someone logging sleep and stress and wearing a watch, phase is about a third of the picture, not the whole of it.

But I am not going to pretend 35% is a number the research handed me. It is a number I chose, not one a study gave me, and it is the open question I am working on. When the weight changes, the app will show the new weight. That is the point of showing the math.

What the score is not

The Fe Score does not predict your performance. In the 2026 study, the thing that predicted how the lifting went was motivation to train, not a readiness rating. The score is a morning summary of what you logged and where your cycle probably is, turned into one call. If it says Go and you feel flat, trust flat. If it says Rest and the bar is flying in your warm-up, lift.

It is also not a medical device. The app’s own fine print says so and I will say it here too. It does not diagnose anything, and it is not a substitute for noticing how you feel.

The Fe Score itself is free. Its full history, so you can look back across cycles and see whether the bands matched your real days, is part of Pro. That history is the part I find most useful, because it is how you find out whether the score is right about you.

This is general information, not medical advice. Talk to your doctor about your own health.

Ferrabelle is in a closed test on Google Play right now. If you want to try it, you can join at ferrabelle.com/testers.

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