Fertile Window Calculator

The window runs from five days before estimated ovulation through the ovulation day. We estimate ovulation as 14 days before your estimated next period, using your entered cycle length. Both ends of the displayed date range are included.

No. These dates are estimates based on the calendar, not measurements of your fertility. Actual ovulation can fall outside the forecast, so unhighlighted days must not be treated as safe days for contraception.

The calendar repeats one average cycle length and accepts 21–35 days. If your cycles vary or fall outside this range, do not treat the window as reliable or enter a substitute length. Hormonal contraception and changes after pregnancy can also make these estimates unsuitable.

Green marks the estimated fertile window, with the estimated ovulation day highlighted in lavender. Dark rose shows your entered last period start; light pink shows estimated future period starts. Only the entered cycle and following estimates are shown, not predictions for earlier cycles.

Ovulation is the event when an ovary releases an egg. The fertile window describes the days around that event when sex could lead to pregnancy. This calculator highlights a six-day estimated window, rather than six separate ovulation days. The actual timing can differ from the dates shown.

Sperm can survive in the reproductive tract for up to five days, so sperm from sex before ovulation may still be present when an egg is released. The egg usually remains capable of fertilization for about 12–24 hours. This explains the broader window, but does not establish safe days outside the calendar highlights.