The best one is honest about what it doesn’t know. On iPhone we’d pick Plum, which we make. When your cycles vary a lot it predicts from your last three, one odd cycle doesn’t drag the forecast, and it shows how much your cycles vary in days. Clue covers irregular cycles in the most depth, Apple’s Cycle Tracking can flag a possible cycle deviation, and Drip marks extra possible start days. No app can tell you why your cycles are irregular. If they’re often longer than 35 days or swing by more than about 8 days, see a clinician.
How we chose
We make Plum, and it’s on this list. There are no affiliate links. We read each company’s own pages on October 12, 2026, and every claim about another app links to its source below. Where a page didn’t say something, we say “not stated”.
Irregular cycles change what matters. We looked for four things. Does the app show how sure it is? Can one odd cycle throw it off? Does it make symptom logging easy enough to keep up? And does it give you something a clinician can read?
Plum, for a forecast that follows your recent cycles
Plum is an iPhone app. It predicts your next period, fertile window and ovulation, and how it predicts is built for cycles that move. Until you’ve logged three cycles it uses a weighted average that blends in each new cycle. From three on, it takes the median of your last six cycle lengths, so one strange month barely shifts it. When your recent cycles vary by more than about five days, it switches to the median of the last three, so the date follows where your cycle is now. The calendar still shows one predicted day. Insights shows how much your cycles vary, as plus or minus a number of days, so you know how far to trust it.
One design choice cuts both ways. If a cycle runs more than a week past your usual length, Plum treats it as a possible missed log and leaves it out of the median. That stops one forgotten period from stretching every forecast after it. It also means a genuinely long cycle moves the forecast less than you might expect.
Logging a day takes a tap or two, across 27 things including flow, pain, discharge, mood, sleep, and ovulation and pregnancy tests. On a hormonal birth control method, Plum hides ovulation and the fertile window by default, and you can turn them back on. Logging, predictions and reminders come with the app at no cost, and Premium adds insights. Plum has no free-text notes box and no Android version, and it doesn’t sync with Apple Health.
Clue, for irregular cycles in depth
Clue runs on iPhone and Android, and its help centre covers uneven cycles in more detail than the others here. It explains that predictions draw on your last 12 cycles and averages on your last 6, and it counts back 13 days from your next period to estimate ovulation. A positive ovulation test or temperature data can change that estimate. Three past cycles are enough for its core predictions, though it says irregular cycles may need more.
If a pregnancy, emergency contraception or a skipped period has thrown one cycle off, you can hide that atypical cycle from Clue’s calculations. Its home page also offers a No Period mode, which starts with a free account, for tracking without bleeding. A Clue Plus subscription adds a perimenopause mode, cramps predictions, BBT charts and wearable temperature sync.
Apple Cycle Tracking, for a built-in warning
Cycle Tracking in the iPhone Health app can send a Cycle Deviation notification when your logs show a possible pattern. Apple names irregular, infrequent and prolonged periods and persistent spotting, and says they may point to an underlying condition or to other factors. You review your history from the notification, and Apple suggests next steps. You can export your cycle history as a PDF for an appointment. You can also log factors such as pregnancy, lactation and contraceptive use, which Apple uses to manage your predictions. It costs nothing and is already on your iPhone.
Drip, for seeing the spread of possible dates
Drip is an open-source app that keeps everything on your device. Its FAQ explains its calendar plainly. After three complete cycles it predicts your next three period starts. It marks the most likely start day with one solid circle and adds dashed circles for other possible days, two of them when your cycles are steady and four when they vary more. That’s a clear picture of uncertainty. Its FAQ is also frank that it skips cycles longer than 99 days, which it says isn’t ideal for some people with PCOS or in perimenopause, and that it can’t pause tracking yet.
Natural Cycles, if you need birth control
None of the apps above is contraception, and an irregular cycle makes any date-based fertile window shakier. Natural Cycles calls itself the only FDA-cleared birth control app and works from your temperature and other body signals. Its home page also lists a perimenopause mode. How it handles very irregular cycles is not stated there, so ask a clinician whether it suits you.
Side by side
| Shows uncertainty | Handling an odd cycle | Runs on | Cost | |
|---|---|---|---|---|
| Plum | Cycle variation in days, shown in Insights | Median of recent cycles, and a cycle over a week late is left out | iPhone | Predictions at no cost, Premium for insights |
| Clue | Says irregular cycles may need more data | Hide an atypical cycle yourself | iPhone, Android | Free version, Clue Plus for more |
| Apple Cycle Tracking | Cycle Deviation notifications | Log factors such as contraception | iPhone, Apple Watch | Built in |
| Drip | Extra possible start days on the calendar | Skips cycles over 99 days | iPhone, Android | Free and open source |
| Natural Cycles | Not stated | Not stated | Not stated | Not stated |
What to look for when your cycles are irregular
- Choose a range over a single date. On an uneven cycle, a window is the honest answer.
- Log symptoms on the day. Pain, flow and discharge noted as they happen are what a clinician can use later. Our period diary guide covers what to note.
- Check how the app treats one odd cycle. A median, or a way to hide the cycle, keeps one bad month from skewing the rest.
- Don’t lean on the fertile window. It’s worked out from the period forecast, so it inherits every bit of its doubt.
When to see a clinician
A tracker can show you a pattern, but it can’t explain it. Book an appointment if your cycles are often longer than 35 days or shorter than 21, or if they vary by more than about 8 days from one cycle to the next. Go too if your periods stop for three months and you’re not pregnant, or if you bleed between periods or after sex. Bring a few months of logged dates. Our guide to what causes irregular periods and the irregular period calculator can help you read what you’ve logged.
Sources
- Plum Period Tracker & Diary on the App Store
- Clue home page
- Clue, How are my predictions and averages calculated?
- Clue, How long would it take Clue to give accurate predictions?
- Clue, What if an atypical cycle is making my Clue predictions inaccurate?
- Clue, What’s included in Clue Plus?
- Apple, View menstrual cycle predictions and history in Health on iPhone
- Apple, Log menstrual cycle information in Health on iPhone
- Drip home page and Drip FAQ
- Natural Cycles home page
