Flo vs Clue: What Each One Publishes About Your Data

This page compares two documents, not two experiences. It cannot install either app, cannot inspect either company's servers, and cannot tell you which one is better — so it does not rank them. What it can do is put what Flo and Clue each publish about your data next to each other, in their own words, and mark the places where one of them publishes nothing at all. Clue's policy, last updated 14 July 2026, says where data is stored and that health data is never shared with or sold to advertisers. Flo's policy, effective 9 April 2026, says it does not sell or rent personal data for money and publishes a separate Anonymous Mode description. Those are the checkable statements, and the rest of this page is them.

The restriction is the point. Every comparison in this genre is written by somebody who used both apps for an afternoon and then made confident claims about what happens on servers they never saw. Nothing here rests on that. Where a reader would want a fact and no published document supplies it, the cell below says so.

What each company publishes

Two cells say nothing rather than something. A comparison that filled them in from a review, a screenshot or a memory would look more useful and be worth less.

How to read a table like that

Three things are worth saying about it, and they apply to any comparison of published policies.

The first is that silence is not evidence. Where a cell above says nothing is stated, that means this page cannot check it, not that the company does the opposite. Policies are written to cover legal ground rather than to answer the questions a reader has, and a missing sentence is usually an editorial choice about scope.

The second is that the sentences are not the same shape. "Does not sell or rent personal data for money" and "health data is never shared with or sold to advertisers" cover overlapping but different ground: one is about money, the other about a category of recipient. Comparing them as if they were the same claim would be the sort of sleight this page exists to avoid.

The third is the dates. Clue's policy carries a last-updated date of 14 July 2026 and Flo's an effective date of 9 April 2026. Both are current at the time of writing and both can be replaced at any time, without notice to you. Anything you decide on the strength of a policy is a decision about a document that can change next week. Check the date on the version you are reading before you trust a summary of it, including this one.

What regulators have on the record

The public record matters more than either company's own account of itself, because it is written by somebody with no stake in the answer. There is not much of it, and the wording carries weight.

In January 2021 the US Federal Trade Commission announced a settlement with the developer of a widely used fertility-tracking app. The FTC's complaint alleged that the company promised to keep users' health data private while disclosing it to third-party analytics providers. The company settled, neither admitting nor denying the allegations, and agreed to obtain express consent before sharing health data in future. Every word in that sentence is doing work: an allegation is not a finding, a settlement is not an admission, and the matter is closed.

The 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. That is a regulator looking at the whole category and coming away without an enforcement action.

One settled matter and one review finding nothing serious is not a league table. Anyone using either as proof of what a company does today is doing something the documents do not support.

The thing neither company can fix

Both apps get judged, in reviews and in arguments, on whether their predictions are right. That is the least fair test in the genre, because the limit is in the biology rather than in either company's code.

A study of real-world cycle data found that ovulation occurred most commonly on day 15, in 27% of cycles, then day 16 in 21%, then day 14 in 20%. Even within a 28-day cycle, the observed ovulation days spread across ten days. The authors concluded that accurate prediction from calendar input alone was not possible.

Read that carefully, because it is the whole argument. It is not that some apps are bad at this. It is that the information a calendar holds does not contain the answer. An app given nothing but your start dates is being asked a question its input cannot settle, and the honest output is a window with the uncertainty attached rather than a day with a confident icon on it. The same ceiling sits over every tracker on the market, and over Athena.

You can see the shape of it yourself in about a minute. Put your last few start dates into the cycle length calculator and it will show your own average and how much it moves — the second number is the one that decides how wide any honest prediction has to be. Feed those into the period calculator and it gives you a window with the uncertainty printed next to it, which is what both apps are doing underneath whatever their screens look like.

The question of cost

People arrive at this comparison wanting a price and this page does not carry one. Prices change, they differ by region, and a figure typed once would sit here misleading readers for years. Each app's own store listing is the only current answer, and it is the only source worth trusting on it. This page can compare dated documents, and a price is not one.

Decide it for yourself in ten minutes

You do not need a verdict from a website. You need four checks you can run on your own phone, and they beat any comparison including this one.

1. Open the policy and read the date. If it is older than the app's last big release, you are reading about a previous version of the product.
2. Search that page for the word "advertising". Read every sentence it appears in, and note whether the promise is about selling, about sharing, or about a named category of recipient. They are different promises.
3. Find the deletion clause and time it. Note what has to be deleted separately, where the request is made, and how long the company says it takes. An app that names its backups is telling you more than one that does not mention them.
4. Turn the connection off and use it for a day. If logging, history and predictions all still work in airplane mode, the arithmetic is on the device. This is the only test on the list that produces your own evidence rather than somebody's sentence.

The ten-minute privacy check is the long version of that list, with the exact things to look for in each clause. If you are leaning towards leaving one of them, the honest field of Flo alternatives and what actually changes if you leave Clue take it from there, and the difference between a promise and an architecture is the argument underneath all three. If you would rather keep the whole question off a phone, the printable period tracker has no policy to read, because there is no company.

Where Athena stands

Athena is the app this site is being built for, and it is not in this comparison for a simple reason: it is in development, it is not in any store, nothing here is for sale and there is nothing to sign up for. Putting an unreleased app in a table next to two shipped ones would be the kind of thing this page was written against. What exists today is the free tools on this site and the writing.

When it ships, cycle data is written to a private database on the phone, predictions are calculated on the device, and there is no account and no server copy of health data. Two things still leave: anonymous diagnostics to Firebase, meaning which features get used, crashes and device model, never anything you log, and this website's IP-anonymised web analytics. And local-only cuts both ways — there is no automatic backup, so if the phone is lost and nothing was exported, the history is gone. When it does ship, the honest way to check any of that will be check four above, run on it.

If neither is what you are after, the alternatives to each are set out in Flo alternatives and Clue alternatives, and the case for an app whose code anybody can read is in open-source period trackers.