Which phase of your cycle are you in?
Enter the first day of your last period and your usual cycle length. You'll get an estimate of your cycle day, your phase and when your next period may start.
Runs only in your browser: nothing you type is sent or saved. An estimate for understanding your body, not a medical tool and not a way to prevent pregnancy.
How the estimate works
A cycle starts on the first day of your period. The phase before ovulation (follicular) varies a lot from person to person, while the phase after it (luteal) is usually closer to two weeks. So the calculator places ovulation about 14 days before your next expected period, and counts the phases around it.
| Phase | Inner season | Roughly when |
|---|---|---|
| Menstrual | Inner winter | Your period days |
| Follicular | Inner spring | After your period until a few days before ovulation |
| Ovulatory | Inner summer | A few days around ovulation |
| Luteal | Inner autumn | From ovulation until your next period |
Why it's only an estimate
Cycles vary. A large study of over 600,000 cycles found that most people's cycle length changes from month to month, and that ovulation day varies too. That's why your next period is shown as a range, not a date, and why this page will never tell you that a day is "safe". Read more in why period predictions are ranges.
If your cycles are often shorter than 21 days or longer than 35, change a lot, or your period stops, it's worth talking to a doctor.
Learn your real pattern
One date and an average only go so far. Twilune learns from your own logged cycles, shows each prediction with a confidence level, and tells you how your phase tends to feel for you: energy, mood and focus.