From your own past cycles, and honest about how sure it is.
This is the same text the app shows under Settings › How estimates work.
Your next period
- 12
- of your most recent cycles, at most, go into the estimate
- 3
- cycles before there is any estimate at all
- 15–60
- days: the cycle lengths it counts
The next period is estimated from the lengths of your own recent cycles, up to the last twelve. The likely day is the middle of those lengths.
The range around it is honest about how much your cycles vary. With three to five cycles it runs from your shortest to your longest, plus a day either side. With six or more, it covers the middle eighty percent.
Cycles shorter than 15 days or longer than 60 are left out, and so is any cycle you mark as unusual. The app says when it has left some out.
Under three cycles there is no estimate. A guess from one or two cycles would look more certain than it is.
Fertile days
- 0.2 °C
- rise in waking temperature, held for three mornings
- 1–2
- days from a positive ovulation test to ovulation, usually
From the calendar
Fertile days from the calendar alone are a wide estimate. They are not contraception.
From your temperature
If you log your temperature on waking, a rise that holds for three mornings, the last at least 0.2 °C (about 0.4 °F) above the six mornings before, usually means ovulation has passed. Once that has happened in three or more cycles, the app learns how long your cycles run after it, and uses that to narrow the next-period estimate.
From an ovulation test
A positive ovulation test usually means ovulation within a day or two. It narrows the fertile days for that cycle, but on its own it doesn’t move the next-period estimate, since a surge can happen without ovulation.
From an Apple Watch
An Apple Watch measures your wrist while you sleep. Those nights are noisier than a thermometer on waking, so the app looks for a rise of at least 0.2 °C that holds until your period, and places it within a few days rather than to the day. It never names an ovulation day from the watch alone. When it sees a rise, it widens the fertile days around it, and narrows the next-period estimate only when that’s clearly narrower than the one from your cycle lengths. Waking temperatures, when you log enough of them, always decide over the watch.
Neither is a guarantee. A temperature rise can also come from a short night, illness or a late morning.
What it isn’t
It isn’t contraception, and no day is safe to rely on. It isn’t a medical device and doesn’t diagnose anything. Estimates come from your own past cycles and can be wrong. For anything about your health, talk to a doctor or nurse.