A free, offline charting app for the symptothermal method (STM / NFP / FAM). Android. No account. No ads. No predictions dressed up as science.
I'm a med student. STM came up during my gynecology module, and I'd vaguely heard of it before but always assumed it was the rhythm method with extra steps. Turns out it's a genuinely well-researched method with clearly defined rules — the rotation made me actually read into it properly.
Not long after, my partner told me she wanted to come off the pill. So we figured we'd try STM together. She read through the method literature, I had the background from uni, and we started charting.
Then came the app question. I spent a weekend going through what's out there and got progressively more confused. A lot of apps advertise STM but when you look at how they actually determine the fertile window, it's based on your past cycle lengths — which is the calendar method with a temperature graph on top. The ones that do implement STM correctly are either behind a subscription, German-only, or feel abandoned.
I didn't want to pay a monthly fee for what's essentially a digital version of a paper chart. And I wasn't comfortable using something where I couldn't tell what logic was running underneath.
So I spent a few weekends building one. It turned out okay, so I figured I'd put it out there.
A charting tool. BBT, cervical mucus, cervix, the usual. The evaluation follows the published STM rules and the AG NFP guidelines — nothing else.
- No algorithm that learns from your data.
- No predictions from past cycles.
- No proprietary logic.
If a day gets marked fertile, you can trace exactly which rule caused it.
There's an extensive section on the biology behind the method — the hormonal cycle, why the biomarkers work, how the rules were derived from the underlying physiology. Every claim in there is backed by primary sources.
For me that's as important as the charting itself. Ideally you learn STM from a qualified instructor, but I wanted people to have the option to understand the method properly rather than just following rules on faith.
- Everything stored locally on your phone.
- No account, no sign-in, no cloud.
- No analytics, no tracking, no ads.
- Nothing leaves the device.
Grab the latest APK from the Releases page.
Android only for now. iOS is on the maybe-list.
This is a first release. The charting logic works and has been checked against the guidelines, but the UX has rough edges and I'd rather hear about them than pretend they aren't there.
Useful places to tell me:
- Open an issue for anything concrete — bugs, rule interpretations that look off, UX friction, missing inputs.
- Discussions for open-ended questions or ideas.
If you've been charting a while and something in the evaluation doesn't match the guidelines as you understand them, please say so. That's the kind of feedback I want most.
Crest is a charting tool, not a medical device. It does not provide medical advice, diagnosis, or treatment, and it is not a substitute for a qualified instructor.
The symptothermal method is effective when applied correctly by someone trained in it. If you are using STM to avoid pregnancy, learn the method properly before relying on it.
This app is an independent project. It is compatible with the symptothermal method as taught by the AG NFP / Malteser, but it is not affiliated with, endorsed by, or certified by any organization. "Sensiplan" is a registered trademark of the Malteser AG NFP and is not used to describe this app.
Built with Flutter. Local SQLite, no backend.
flutter pub get
flutter runThe STM evaluation engine lives in lib/services/stm_engine.dart.
If you spot a rule that's implemented incorrectly, that's where to look —
and a PR with a failing test case is the fastest way to get it fixed.
MIT. Do what you want with the code.
The method itself belongs to nobody. The trademarks belong to their owners.