Period Calculator
Forecast your next periods from your cycle history.
Cycle details
days
days
Pick the first day of your last period.
Averages and variation
The textbook cycle is 28 days, but 21–35 is normal for adults, and a few days of variation month to month is expected. Projections here simply add your average cycle length — three cycles of tracking makes the average trustworthy.
When to check in
Cycles consistently shorter than 21 days, longer than 35, bleeding beyond 7 days, or sudden pattern changes are worth a clinician's look — as is a missed period with a negative test repeated a week later.
Frequently asked questions
- How does the Period Calculator work?
- Forecast your next periods from your cycle history.
- Is the Period Calculator free to use?
- Yes. The Period Calculator is completely free, runs entirely in your browser, and requires no account or sign-up. Your inputs never leave your device.
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