A friend was asking, and it would be nice to have one or two to recommend.
Bonus if it's on Google Play in addition to Fdroid, but not necessary.
Some recommendations in these threads:
Some discussion on apps (including non-FOSS) here:
Leading Recommendation from the comments
The leading recommendation seems to be Drip (bloodyhealth.gitlab.io)
Summarizing what people shared:
- accessible: it is on F-droid, Google Play, & iOS App Store
- does not allow any third-party tracking
- the project got support from "PrototypeFund & Germany's Federal Ministry of Education and Research, the Superrr Lab and Mozilla"
- Listed features:
- "Your data, your choice: Everything you enter stays on your device"
- "Not another cute, pink app: drip is designed with gender inclusivity in mind."
- "Your body is not a black box: drip is transparent in its calculations and encourages you to think for yourself."
- "Track what you like: Just your period, or detect your fertility using the symptothermal method."
Their Mastodon: https://mastodon.social/@dripapp
Also drip seems to be supported by the German ministry of education and science. Usually Germans rank privacy rather high in importance. So I think that speaks in favor of it too.