Ovulation & Fertile Window Calculator
Enter your last period start and cycle length to see next periods, estimated ovulation and fertile windows on a calendar, using the 14-day luteal method or the rhythm method.
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How to use
Everything you enter is processed only in this browser.
Choose the first day of your last period
Enter your average cycle and period length
See your expected periods, fertile window and ovulation on the calendar
How it works and good to know
Next period = first day of last period + average cycle; expected ovulation = next period − 14 days; the fertile window runs from 5 days before ovulation to the day after (14-day luteal phase method). If you choose shortest and longest cycle, the rhythm method (Ogino-Knaus) counts days (shortest − 18) to (longest − 11) of the cycle as fertile. Cycles of 21–45 days and periods of 2–10 days are accepted, and 3 or 6 cycles are shown. Actual ovulation can vary from cycle to cycle, so this can’t be used as contraception and doesn’t replace a diagnosis. Dates are calculated only in this browser and not saved, and the calendar file (.ics) is created only when you press the button.