Open Source Period Trackers: What Being Open Actually Buys You

Open source buys you one thing, and it is a good thing: the promise in the privacy policy can be checked against the code by somebody. That is a different kind of assurance from a company saying trust us. It is not proof. Most people cannot read code, published source is not evidence that the build in the store was made from it, and an open licence says nothing about whether anyone is still maintaining the project. The two open-source cycle apps people ask about are drip and Euki, and everything below about either of them comes from what they publish about themselves.

So the useful question is not "is it open source?" but "what does being open let me, or somebody like me, actually verify?" The answer is narrower than the word suggests and still worth having.

What being open actually buys you

Every closed app asks you to accept a claim. The policy says the data stays on the device; you have no way to see whether it does. Open source changes the category of that statement from a promise to something falsifiable. If the code says otherwise, someone can say so publicly, and the gap between the document and the behaviour becomes a fact rather than a suspicion.

The drip privacy policy makes exactly this argument in its own words: it states that the source code can be read to check that what the policy says is true. That sentence is the whole case for open source in one line, and it is an unusually honest thing for a policy to say, because it invites the reader to go and look rather than asking to be believed.

The second thing it buys is slower and less obvious. An open project cannot quietly change what it does. A closed app can ship a new version that starts sending something new, and the only trace is a line in a policy nobody reads. In an open project that change is a public diff with a date on it.

Four things it does not buy you

1. You cannot read it. Almost nobody can. What open source really gives you is the possibility that somebody competent looked — a researcher, a journalist, a contributor with a grudge. That is genuine and it is second-hand. You are still trusting people, just a wider and less interested set of them.
2. Published source is not the app on your phone. The code in a repository and the binary in a store are two different objects. Making the second provably follow from the first is a hard problem with a name, reproducible builds, and most projects have not solved it. So "the code says it stores nothing" and "this installed app stores nothing" are separate statements.
3. Open says nothing about maintained. A repository that has not been touched in three years is still open source. A cycle app that stops getting updates keeps working until an operating system release breaks it, and nobody is obliged to fix it. The licence is not a commitment.
4. A nonprofit can still close. Being run by a foundation or a charity rather than a company changes the incentives, which is real. It does not make the project permanent. Funding ends, volunteers move on, and the app that was there when you started tracking may not be there in five years.

None of that is an argument against open-source apps. It is an argument against treating the word as a conclusion.

What drip publishes about itself

The drip privacy policy states that there is no collection of usage data or personal information, no ads and no spyware. It states that menstrual cycle data is stored locally on the device and cannot be accessed by other apps, and that deleting the app data in settings, or uninstalling the app, removes it.

The most interesting sentence in it is a caveat the project did not have to include: the policy states that an automatic cloud backup may still take place depending on your device settings. That is a limit of on-device storage in general rather than a flaw in drip, and it is the sort of thing marketing usually leaves out. A phone that backs itself up to a platform cloud is backing up what is on it. An app cannot always know or override that, and a policy that says so is telling you something true about the world rather than something flattering about itself.

What Euki publishes about itself

Euki's privacy policy, effective June 2024, states that it does not collect any personal or other information from users, including through their use of the app, except for information a user voluntarily sends by contacting support. It also states that the app does not track its users to provide targeted advertising.

Euki describes itself as an open-source project managed by an independent 501(c)(3) nonprofit, funded by community support and donations, free to use with no paywalls, and says that because it never collects personal data it does not sell it or use it to target advertising. The funding model matters more than it looks: an app with no advertising and no paid tier has no revenue reason to want your data, which removes the pressure that creates most of the behaviour the rest of this site writes about.

Both descriptions above are the companies describing their own products. We have not audited either app, and this site does not repeat a claim about a named app that the company has not published itself.

The market both of them sit in

Two pieces of context, because "open source is safer" is only meaningful against a baseline.

The baseline is not reassuring. A peer-reviewed scoping review of health apps found that most of those examined — 20 out of 23 — shared user data with third parties. That is the genre, not a specific accusation, and it is the reason the question gets asked at all.

The honest counterweight belongs in the same breath. When the UK's Information Commissioner's Office reviewed period and fertility apps in 2024, it reported that no serious compliance issues or evidence of harms were identified, while urging developers to prioritise privacy. Nobody is entitled to quote only one of those two findings.

The legal floor is lower than most people assume, whichever app you pick. The FTC's own guidance says many companies collecting health information — a fitness tracker, a diet app, a connected blood pressure cuff — are not covered by HIPAA; the FTC Act and the Health Breach Notification Rule apply instead. An app you found and installed yourself is usually in that second category, and the Health Breach Notification Rule is largely a rule about being told afterwards.

How to check any of it without reading code

The tests that work on a closed app work on an open one, and you do not need a compiler for any of them. Turn the phone to airplane mode and use the app for a day: if logging, history and predictions all still work, the calculations are happening on the device. Look for whether it opens straight into the tracker with no email, which is the difference described in the no-account comparison. Then find the export, and use it once, before you have a year of data to lose.

That last one is where an on-device app of any licence is weakest, and it is worth being blunt about. Data that never leaves your phone is data with no automatic backup. Lose the phone with nothing exported and the history is gone. The trade is real and it is the subject of the offline-versus-cloud piece, which is the other half of this decision.

Two things on this site help with the same problem from the outside. The printable period tracker gives you a paper copy that no app version, licence or funding round can take away, which is also the best format to carry into an appointment. And the cycle length calculator works out your own average cycle and how much it moves from a handful of start dates — so if a tracker ever disappears, the number that mattered is one you already know. Both run in your browser and keep nothing.

Where Athena sits

Straight answer: Athena is not open source, so it does not get the assurance this article is about, and pretending otherwise would be the exact behaviour the page is written against. It is also in development, is not in any store, and has nothing to sign up for.

What it offers instead is the architecture: cycle data written to a private database on the phone, predictions calculated on the device, no account and no server copy of health data. The app will send anonymous diagnostics — features used, crashes, device model, never anything you log — to Firebase, and this website uses IP-anonymised web analytics, so "sends nothing" would be false. And it carries the same cost as every on-device app here: no automatic backup, so an export you actually made is the only insurance.

You will not be able to verify that by reading our code. You will be able to verify it the way the privacy checklist describes: turn the connection off, and see what stops working.