Plum predicts from the periods you log, and only from those. It doesn’t read your temperature, your heart rate or a test strip. Every period you add makes the next prediction a little better, and this page says exactly how.
Your next period
Before you’ve logged three cycles, Plum starts from the cycle length you gave it during setup. If you skipped that, it uses a typical length for your age, which runs from about 28 to 31 days across age groups. Each cycle you log then nudges the estimate towards your own rhythm.
From three logged cycles on, the prediction is the median of your recent cycles, up to the last six. A median ignores one odd cycle instead of being dragged by it. A cycle that runs more than a week past your usual length is treated as a possible late log and left out of the median too, so one forgotten period doesn’t stretch every forecast after it.
When your cycles vary by more than about five days, Plum switches to the median of the last three and shows a prediction window instead of a single day. That’s the honest answer when the data is uneven, and it’s why Plum says it’s still learning rather than guessing. The irregular periods guide covers what makes cycles swing and when it’s worth a doctor’s visit.
How long your period lasts
The predicted length is the median of your last three periods, or the length you gave during setup before there are three. When you log the first day of a period, Plum fills in the rest of the usual days for you, and you can tap any of them to correct it. A gap of up to two unlogged days inside a period still counts as the same period.
Ovulation and the fertile window
Plum counts back from the next expected period. The stretch from ovulation to the next period, the luteal phase, is taken as 14 days for cycles of 25 to 32 days, a little less for shorter cycles and a little more for longer ones. The day that lands on is the estimated ovulation day. The fertile window is the five days before it plus the day itself, six days, because sperm can live about that long.
That’s an estimate from dates, and it carries the same uncertainty as the period prediction it hangs off. Our fertile window guide explains what the estimate can and can’t tell you, and why it isn’t contraception.
Birth control
If you set up a hormonal method in Me, then Birth Control, Plum lowers its confidence, softens its wording, and turns the fertile window and ovulation display off by default, because those methods change or stop ovulation. Period logging and period predictions keep working the whole time.
Common questions
How many cycles does Plum need before predictions settle?
Three. Until then Plum leans on the cycle length you gave it during setup, or a typical length for your age if you skipped that, and blends in each new cycle as it arrives. From the third logged cycle on, the prediction is the median of your recent cycles, up to the last six.
Why does Plum show a range instead of a date?
Because your recent cycles vary by more than about five days. When that happens Plum switches to the median of your last three cycles and shows a prediction window rather than one confident day. It narrows again once your cycles do.
How does Plum work out my ovulation day?
It counts back from your next expected period. For a cycle of 25 to 32 days it takes ovulation as 14 days before the next period, and it adjusts that gap for shorter or longer cycles. It’s an estimate from dates, not a measurement, and it doesn’t read temperature or test strips.
What is the fertile window in Plum?
The five days before the estimated ovulation day, plus the day itself, six days in all. That reflects how long sperm can survive. The window is only as good as the ovulation estimate behind it.
Can I use Plum’s fertile window to avoid pregnancy?
No. The fertile window and ovulation day help you understand your body, and they are not reliable enough to prevent or plan a pregnancy. Plum is a wellness app, not a medical device or a form of contraception.
Why did my predictions change when I set up birth control?
Hormonal methods, such as the pill, patch, ring, shot, hormonal IUD or implant, change or stop ovulation, so Plum lowers its confidence and turns the fertile window and ovulation display off by default while a hormonal method is active. Period logging and period predictions keep working.
I forgot to log a day mid-period. Does that split it into two periods?
No. A gap of up to two unlogged days between period days counts as the same period. Longer gaps start a new one.
