Period & Ovulation Calculator
Next period, fertile days & ovulation
| Period starts | Fertile window | Ovulation |
|---|
Predictions assume a regular cycle and ovulation about 14 days before the next period. Cycles vary, so don't rely on this for contraception. See a doctor about very irregular or painful periods.
About the Period & Ovulation Calculator
Know when your next period is due and which days you're most fertile. Enter the first day of your last period, your usual cycle length and how long your period lasts, and see your next six periods, your fertile window and ovulation day on a calendar. Everything stays on your device.
- Pick the first day of your last period.
- Enter your average cycle length (from the first day of one period to the first day of the next) and how many days your period lasts.
- Read your next period, ovulation and fertile window, and browse the calendar.
Frequently asked
How is the next period calculated?
By adding your cycle length to the first day of your last period. For a 28-day cycle starting 1 March, the next period is due on 29 March.
When do I ovulate?
Ovulation usually happens about 14 days before the next period, so on day 14 of a 28-day cycle and day 21 of a 35-day cycle. The fertile window is the five days before ovulation plus ovulation day and the day after.
Can I use this to avoid pregnancy?
No. Cycles and ovulation vary from month to month, and sperm can survive for up to five days, so calendar predictions aren't reliable contraception.
What if my cycle is irregular?
Use your average cycle length over the last few months; predictions will be less exact. Very irregular, missed or painful periods are worth discussing with a doctor.
Is what I type sent to a server?
No. The tool runs entirely in your browser and there's no server behind it, so what you type never leaves your device and nothing is stored. You can check: it keeps working if you go offline after the page has loaded.
More from the Calculators drawer
Other tools kept in this drawer, plus a few favourites from the bench.