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.
Export everything to a plain, readable file, any time you like.
SETUP TAKES ABOUT FORTY SECONDS
A few questions up front: when your last period started, how long your cycle runs, how many days you bleed, what you care about tracking and, if you know them, your hardest days. After that, logging is optional. Undertow keeps working whether you open it.
ONE PAYMENT
Paid once. No subscription, no renewal, no in-app purchase, no upgrade tier. Nothing to cancel and nothing to forget about.
ABOUT YOUR DATA
Everything lives in a single file on your iPhone, encrypted at rest and unreadable while the phone is locked. It moves to a new phone through your normal encrypted iPhone backup, or through a backup file you export yourself.
We never hold a copy. Not encrypted, not anonymised, not aggregated. We could not hand one over if somebody asked us to, because there is nothing to hand over.
Made by a woman, for women.
Predictions are estimates based on the cycles you have logged. Undertow is not a contraceptive method and does not provide medical advice. If something about your cycle worries you, talk to a doctor.
