No period tracker app has strong independent evidence that it predicts period dates better than the others. Every calendar app works from the same input, the start dates you log, so on a steady cycle they agree within a day or two and on an uneven cycle they all miss. Natural Cycles is the one with FDA clearance and clinical studies behind it, but that’s for finding fertile days from your temperature, not for forecasting your period. The most accurate app for you is the one you log carefully for several cycles and that’s honest about how much your cycle varies.
How we chose
We make Plum, so read our section on it with that in mind. There are no affiliate links on this page. We read each company’s own pages and App Store listings on October 12, 2026, and we only cite research papers we could open and read in full. We looked for an independent test that compares current apps on period-date accuracy and didn’t find one. So this page doesn’t crown a winner. It explains what drives accuracy and what each app uses to predict.
What the research says
The biggest look at real cycles comes from app data. A 2019 study in npj Digital Medicine analysed 612,613 ovulating cycles from 124,648 Natural Cycles users. The average cycle was 29.3 days, and only 13% of cycles were exactly 28 days long. The days before ovulation varied far more than the days after it, which means ovulation often doesn’t fall on day 14. The study was funded by Natural Cycles and two authors work there.
A 2020 study in the same journal used Clue data from 378,694 users and 4.9 million cycles. About 7.7% of those users had cycles that consistently swung by 9 days or more from one cycle to the next. For them, a single predicted date is a rough guess in any app.
On ovulation, a 2018 pilot study in Frontiers in Public Health scored 12 fertility apps against the evidence. The six calendar-based apps scored zero for predicting the most fertile days. Apps that used temperature and other body signs from the current cycle did much better. That study is about fertile days, and it’s now several years old, but the reason holds. Dates alone can’t see ovulation.
What makes any period prediction more accurate
- Log the first day of full bleeding as day 1, not a day of spotting before it. One wrong start date skews the cycle before it and the one after.
- Keep logging for at least three cycles. Every app starts from a typical cycle length and only learns yours as you add periods.
- Fix missed periods in the history. A period you forgot to log makes the app think one cycle was twice as long.
- Expect less from an uneven cycle. ACOG says cycles of 21 to 45 days are normal in the first years after periods start, and it can take six years or more for them to settle. No app can predict what your body hasn’t settled on.
- Measure ovulation if it matters to you. Basal body temperature or LH test strips show what happened in this cycle. A calendar app only estimates it from past cycles.
Natural Cycles, for fertile days you measure
Natural Cycles says on its home page that it’s the only FDA-cleared birth control app and that it’s clinically proven to pinpoint fertile days. It gives 98% effectiveness when used as intended and 93% with typical use. It works from your temperature, which its site says you can measure overnight with its own band or another wearable. That’s a different job from forecasting your period. If you want to know when you ovulate, it’s the strongest evidence on this page.
Clue, for fertility predictions with Clue Conceive
Clue says Clue Conceive, its mode for trying to get pregnant, uses a clinically validated algorithm to find your most fertile days and is CE marked. That claim covers conception, not period dates. Its App Store listing says its regular period predictions are personalized from your cycle length, flow and patterns, and that Clue shouldn’t be used as a contraceptive.
Flo, for a big app that learns from your dates
Flo’s accuracy page says it combines the cycle dates you’ve tracked with its own AI algorithm. It also says predictions get better once you’ve logged at least three periods. The headline figure on that page, 90%, is the share of users who said Flo predicts their period well. That’s a satisfaction survey, not a measured error rate.
Apple Cycle Tracking, for ovulation estimates from your watch
Apple’s Health app predicts your next period and fertile window from what you log. Apple’s support page says Health can also use Apple Watch data to improve those predictions. With an Apple Watch Series 8 or later, or any Ultra, it marks a day you likely ovulated, looking back after the fact. It can also send a Cycle Deviation notice when your logged history shows irregular, infrequent or prolonged periods.
Plum, for a method you can read
We make Plum, so here is exactly what it does, with nothing held back. Before you’ve finished a cycle, Plum starts from the cycle length you gave it at setup. If you skipped that, it uses a typical length for your age. With one or two finished cycles, it blends each new cycle into a weighted average, so one odd month can’t yank the forecast. From three cycles on, it uses the median of your last six cycle lengths.
Plum also measures how much your cycle varies, as the standard deviation of those recent lengths. The Insights tab shows it as plus or minus a number of days, and until there’s enough history it tells you it needs another cycle. When the variation tops about five days, Plum leans on the median of your last three cycles instead. If one cycle runs more than a week past your usual, Plum treats it as a likely missed log and leaves it out.
Ovulation is counted back from your next expected period, 14 days for a 25 to 32 day cycle and a little less or more outside that. The fertile window is that day plus the five before it. It’s an estimate from dates. Plum doesn’t read temperature, so it isn’t more accurate than a temperature method for ovulation, and it isn’t birth control. Our prediction help page goes further. Plum is on iPhone only.
Side by side
| What it predicts from | What its own pages claim on accuracy | Birth control | |
|---|---|---|---|
| Natural Cycles | Temperature plus period dates | FDA-cleared, 93% typical use | Yes, FDA-cleared |
| Clue | Your logged cycles, flow and patterns | Clue Conceive fertile days clinically validated | No |
| Flo | Logged dates and an AI algorithm | 90% of users surveyed said it predicts well | No |
| Apple Cycle Tracking | Logged dates, plus Apple Watch data if you have one | Not stated | No |
| Plum | Logged dates, median of the last six cycles | No accuracy figure. Shows your variation in days | No |
Sources
- Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, npj Digital Medicine, 2019
- Li et al., Characterizing physiological and symptomatic variation in menstrual cycles using self-tracked mobile-health data, npj Digital Medicine, 2020
- Freis et al., Plausibility of menstrual cycle apps claiming to support conception, Frontiers in Public Health, 2018
- ACOG, Your First Period
- Natural Cycles home page
- Clue Conceive and Clue on the App Store
- Flo, How accurate is Flo?
- Apple, View menstrual cycle predictions and history
- Plum on the App Store
For what the studies found about period apps in general, see how accurate period tracker apps are.
