Why my period tracker predicts a window, not a single day
A period tracker that tells you "your next period starts March 14" is lying to you. Not maliciously — just mathematically. Cycle length varies. For most people it bounces around by a few days each month, and the apps that print one confident little date are quietly averaging that variance out and hoping you won't notice when they're off by four days. When I built my own tracker — Mori , a free,…
Most period trackers claim to predict an exact start date for your next period. However, this is misleading as the length of menstrual cycles naturally varies. The free, offline Android app named Mori addresses this issue by providing a range of dates rather than a single predicted date. The midpoint of this range represents the best estimate, while the surrounding range indicates the level of uncertainty.
Mori's predictions are based solely on your personal cycle history, without relying on crowd data or models trained on other people's bodies. It uses simple statistical calculations to generate the estimate, and the app displays this estimate along with a confidence label. If you log unusual data points, Mori adjusts the prediction's confidence level accordingly, rather than discarding the data.
The app does not attempt to correct any irregularities in your recorded cycle history, but instead acknowledges and records them as they are. This approach ensures that the fertile-window estimate remains an honest representation of your personal cycle, without the pretense of clinical accuracy.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
