Period tracking apps hold some of the most personal information on your phone: cycle dates, symptoms, sexual activity, pregnancy attempts, mood and, for some people, losses they have never told anyone about. Choosing where that record lives is as important as choosing which features you want.
This comparison looks at three very different answers to that question. Flo is the world’s best-known period tracker and runs as a cloud service. Clue is a Berlin-based, science-led app that also stores data on its servers, under European privacy law. Ondi, which we make, keeps everything on your iPhone or iPad and has no server for your health records at all. Each has real strengths, and each asks you to accept a different trade-off.
Cycle data can reveal a pregnancy, an attempt to conceive, a miscarriage or a termination. In places where reproductive healthcare is legally restricted, that information can have consequences far beyond embarrassment. Even where it is not, a leak of cycle data is a different kind of harm from a leak of your step count.
The record so far shows why this matters. In January 2021 the US Federal Trade Commission announced a complaint against Flo Health, alleging that the app had shared users’ health information with marketing and analytics companies, including Facebook and Google, despite promising to keep it private. The settlement, finalised in June 2021, requires Flo to get users’ affirmative consent before sharing their health data and to obtain an independent review of its privacy practices. A California class action followed. Flo and Google settled, and in August 2025 a jury found Meta liable under California’s privacy law for collecting data from Flo users.
None of this means Flo is careless today; the company has invested heavily in privacy since. It does show that a privacy policy is a promise made by a company, and a promise can be broken, rewritten or outlived by the business that made it. The more useful question is where your data physically sits, and who could be asked to hand it over.
Flo is the most widely used period tracker in the world, and it has earned that position. The app is polished and broad: cycle and ovulation predictions, detailed symptom logging, a pregnancy mode, a large library of medically reviewed articles and an AI chat assistant that answers health questions. Because your data lives in Flo’s cloud, it follows you from phone to phone and powers the personalised insights the app is known for.
Flo is built as a cloud service, so your logs are stored on the company’s servers. In September 2022, after the US Supreme Court’s Dobbs decision, Flo launched Anonymous Mode, which lets you use the app without your name, email address or technical identifiers attached to your health data. It was a thoughtful response and remains one of the more serious privacy features in the category. The underlying model stays the same, though: the records themselves still sit on infrastructure Flo controls, now under the terms of the FTC order.
Flo has a free tier for basic tracking. Flo Premium, which unlocks the deeper insights and content, costs around $39.99 a year in the US; prices vary by country and promotions are frequent.
Clue has always presented itself as the evidence-based tracker. Its neutral design avoids the pink-and-flowers look of many period apps, its educational content leans on published research, and the company works with academic researchers on menstrual health. For people who want a calm, data-focused tracker, Clue is an excellent choice.
Clue is based in Berlin, so your data is covered by the EU’s General Data Protection Regulation, which gives you strong rights to access, export and delete it. After the Dobbs decision in 2022, Clue publicly committed to protecting its users’ data. Clue does store your records on its servers, and it uses anonymised, aggregated data in its research partnerships. For many users that research is a reason to choose Clue. If you want no copy of your data anywhere but your own device, it is a point to weigh.
Clue Plus costs about $39.99 a year or $9.99 a month in the US (about £24.99 a year in the UK). It adds predictions up to twelve months ahead, temperature charts, the Clue Conceive, Clue Pregnancy and Clue Perimenopause modes, Clue Connect for sharing your cycle with someone you trust, and temperature integration with the Oura ring.
Ondi starts from a different assumption: the safest copy of your cycle data is the only copy, on your own device. There is no account to create, no server that stores health records and no tracking analytics. The app’s own settings screen lists its network access as “None”. Apple Health is optional: if you allow it, Ondi can read Apple Watch wrist temperature and heart rate and save your periods there, and Delete All Data can also remove what Ondi wrote to Apple Health.
The core tracker is free for good: period tracking with automatic cycle detection, daily logs for flow, mood, energy, sleep and symptoms, cycle predictions, a calendar and alerts when your cycle becomes irregular. Everything is open for the first seven days. After that, a single one-time purchase, with no subscription, unlocks fertility tracking (temperature, cervical mucus and ovulation tests), Trying to Conceive mode, a week-by-week Pregnancy mode with a due-date window, Postpartum and post-menopause modes, perimenopause pattern detection, contraception side-effect matching, headache and migraine patterns, PMS prediction, sleep analysis, exercise guidance for each cycle phase and a data export you can take to a doctor.
A few smaller details follow the same thinking. Lock Screen widgets hide sensitive text, and discreet notifications replace cycle details with a neutral reminder. Ask Ondi lets you ask a question about your own logs and get a short, plain-language answer generated on your device, on devices that support it, clearly labelled as not medical advice. Ondi can also read Colmi-family smart rings directly over Bluetooth for heart rate, blood oxygen and sleep, again without an account.
The trade-offs are real. Because nothing is stored on a server, there is no sync between devices and no account to recover if you lose your phone, so you need to keep your own export. Ondi runs on iPhone and iPad only, with no Android or web version. There is no community and no editorial library on the scale of Flo’s. And like the other two, Ondi is not a medical device and must not be used as contraception.
| Flo | Clue | Ondi | |
|---|---|---|---|
| Where your cycle data is stored | Flo’s cloud servers | Clue’s servers (EU) | On your iPhone or iPad |
| Account model | Account-based; Anonymous Mode detaches your name, email and identifiers | Account-based backup and sync | No account at all |
| Sharing with third parties | Requires your consent under the 2021 FTC order | Anonymised, aggregated data for research | None: no server, no tracking analytics |
| Legal framework | US, with an FTC order in place | EU (GDPR) | Not applicable: no server holds your logs |
| What the company could hand over | Records held on its servers | Records held on its servers | Nothing: the developer has no copy |
| Sync across devices | Yes | Yes | No |
Privacy is only one part of the decision. The feature sets overlap more than they used to, but the differences still matter:
| Flo | Clue | Ondi | |
|---|---|---|---|
| Period predictions | Yes | Yes (up to 12 months ahead with Clue Plus) | Yes (free) |
| Symptom and mood logging | Yes, extensive | Yes, extensive | Flow, mood, energy, sleep and symptoms (free) |
| Fertility signals (temperature, cervical mucus, ovulation tests) | Yes | Yes (temperature charts with Clue Plus) | Yes (with the unlock) |
| Conception and pregnancy | Yes | Clue Conceive and Clue Pregnancy | Trying to Conceive, Pregnancy and Postpartum modes (with the unlock) |
| Content and guidance | Medically reviewed library, AI chat assistant | Evidence-based articles, expert content with Clue Plus | Plain-language insights, on-device Q&A about your own logs |
| Platforms | iOS and Android | iOS and Android | iPhone and iPad |
| Pricing | Free tier; Premium about $39.99/year (US) | Free tier; Clue Plus about $39.99/year or $9.99/month (US) | Free core tracking; one-time unlock, no subscription |
Flo: your history survives a lost phone and appears on any device you sign in to, and the insights draw on a large content and research operation. In exchange, your records exist on a US company’s servers. Anonymous Mode limits what those records say about who you are, and the FTC order limits sharing, but a legal demand or a breach would reach data that sits outside your control.
Clue: the same convenience, with the added protection of EU law, which sets strict rules on how data may be processed and gives you firm rights over it. GDPR regulates access rather than preventing it: a valid court order in the EU can still require Clue to produce records.
Ondi: there is nothing on a server to breach, sell or subpoena, because the developer never receives your logs. Reading them requires your unlocked device, and any device backups you make, such as iCloud Backup, follow the settings you choose. The cost is convenience: one device, manual exports and no account to fall back on.
You want the richest all-in-one experience, with an AI assistant, a deep content library and your history available on any phone. If you choose Flo, turn on Anonymous Mode.
You value a science-led, neutral design and want your data under European privacy law, or you like the idea that anonymised data from your cycles supports menstrual health research. Clue Connect is also a good option if you want to share your cycle with a partner.
Privacy is the deciding factor and you would rather no company held a copy of your cycle at all. Ondi covers the core tracker for free, adds fertility, pregnancy and later-life modes with a one-time unlock, and keeps every record on your device. If you are moving from another app, our guide to switching period trackers explains how to bring your history with you.
There is no single right answer. Cloud-based trackers can do more because they hold your data; on-device trackers do less with it because they never hold it. What matters is choosing deliberately. Read the privacy policy, check the App Store privacy label, and ask one simple question of any app you are considering: if the company were breached, sold or served with a court order tomorrow, what could it hand over? Your answer is the best guide to which tracker fits you.