Undertow: Cycle Tracker

Undertow is a menstrual cycle tracker that never leaves your phone.

No account. No email, no password, no profile. No servers. Nothing is uploaded because there is nowhere to upload it to. No third parties. Zero analytics, zero advertising, zero trackers.

Your cycle is data. Data leaks. So Undertow never takes any.

WHAT YOU SEE EVERY DAY One screen, three facts: which day of your cycle you are on, what today's date is, and which phase you are in. That is the entire daily interaction. Everything else lives one tap down.

WHAT IT DOES Your cycle length is a setting, not an assumption. Anywhere from 21 to 45 days, editable whenever you like. Twenty-eight is an average, not a rule. If your cycles are irregular, Undertow uses a rolling average of the ones you have actually logged instead of a fixed number, once you have logged three.

Drag the ring on the home screen to look forward or back through your cycle. Today keeps its own marker so you never lose your place.

Drag across the calendar to select a stretch of days. Mark it as your period, or tag the whole run at once. Cramps rarely last a single day.

Mark your own hard days. Everyone's fall somewhere different: the first day of bleeding, ovulation, the days before a period, days 19 to 21. Mark them once and they repeat every cycle, on the ring and in the calendar, with an optional heads-up the evening before. Undertow never guesses how you will feel.

Ninety-nine preset tags across fourteen categories, from sleep and skin to mood and medication, all hidden behind one search field so the interface stays empty. Type something that doesn't exist yet and it becomes your own tag.

Write a note on any day.

Patterns tell you where each tag actually lands in your cycle, corrected for phase length so the longest phase doesn't win by default. In plain sentences: "Nearly every cycle, on days 1 to 3." Not a wall of statistics.

Face ID lock, if you want it.

Discovery signals