A Period Tracker That Stays on Your Phone
Period-tracking apps were the first place a lot of women turned to organise their health on a phone. They were also, until recently, one of the least private categories of consumer software — a 2019 Privacy International investigation found that several of the most popular apps were sending menstrual data, fertility status, and even sex-life questions to Facebook for ad targeting. The post-Dobbs landscape made all of that suddenly more consequential.
You don't need to share your cycle data with an ad network to organise it. The math behind a period tracker is simple enough that an entire app's worth of features can run in your browser, on your phone, with zero data leaving the device.
What's actually in a period tracker
The functionality you need from a tracker is small:
- A place to log when each period started and how long it lasted.
- Cycle length history (the gap between consecutive period starts).
- Predictions: when's the next period due, and what phase of the cycle am I in right now.
- A way to see your cycle on a calendar at a glance.
- Optional notes for what felt different this month.
That's it. None of it requires a server, an account, or any data sharing with a third party.
The math, simplified
Your cycle length is the gap between when one period starts and when the next one starts. The "textbook" cycle is 28 days, but anywhere from 21 to 35 is normal, and your own cycle may vary by a few days month to month. Once you've logged at least two periods, the average gap becomes your predicted cycle length — and the next period gets predicted as last start + average cycle length.
Your phase depends on where you are in the cycle:
- Days 1 through your typical period length: menstrual (the bleeding days).
- After bleeding stops up until about 5 days before ovulation: follicular. Estrogen is rising; energy typically climbs.
- The 5-day fertile window through about 1 day after ovulation: ovulatory. Most fertile.
- After that until the next period: luteal. Progesterone peaks; PMS symptoms (when they appear) cluster late in this phase.
Ovulation is roughly 14 days before the next period — the luteal phase length is the more constant of the two halves. Knowing that, you can work backwards from your predicted next-period date to estimate ovulation.
What to look for in a private tracker
If you're shopping around, ask three questions:
- Does the data leave my device? If the app uses an account, syncs to "the cloud", or sends "analytics", the answer is yes.
- Can I export my history? A simple CSV download is the minimum bar. If you can't take your data with you, you're locked in.
- Can I delete everything in one click? No "deactivate account, wait 30 days, then your data is removed" — actually gone, immediately.
The simplest answer to all three is "the data was never anywhere except this browser's local storage to begin with." That's exactly what we built.
Our take
The Period Tracker is one page in your browser. Everything you log is saved to localStorage on this device only. No account, no sign-up, no server, no analytics on the entries themselves. There's a CSV export so you can move your history to your own backup, and a one-click clear-all if you want to wipe it.
Open the tool, log your last period, and the status card immediately shows what cycle day you're on, when the next period is predicted, and what phase you're in. After two periods we have an average cycle to predict from; after three or more, accuracy improves. The tool tells you that explicitly so you don't trust an early prediction more than you should.
A note on what this is and isn't
A tracker is a tracker. It's good for noticing patterns, planning around your cycle, and communicating clearly with a clinician. It's not a contraceptive method, and it's not a diagnostic tool. If your cycle changes suddenly, if you're trying to plan or avoid a pregnancy, or if anything feels concerning, see a clinician. They have context the tracker doesn't.
The short version
Period tracking is small enough to live entirely on your device. If an app is asking for an account before it'll let you log your first period, ask why. The math is public, the code is simple, and your cycle data doesn't belong on someone else's server.