How is the fertile window calculated?

Ovulation is estimated as your next period minus the luteal phase (about 14 days). The fertile window is the five days before ovulation plus the day of ovulation, because sperm can survive several days and the egg lives about a day.

Is a period tracker reliable during perimenopause?

No. Calendar tracking assumes regular cycles. In perimenopause, cycle length varies and ovulation becomes unpredictable, so estimates are unreliable. The tool is educational and is not a contraceptive method.

Period & Ovulation Tracker — Free Calendar Calculator

Period & ovulation tracker

Estimate your next periods, ovulation day, and fertile window from your last period and average cycle length. These are calendar estimates for education — not a contraceptive method.

How the estimate works

The calculator uses the standard calendar method. Your next period is your last period plus your average cycle length. Ovulation is estimated by counting back the length of your luteal phase (the gap between ovulation and your period, usually about 14 days) from that next period. Your fertile window spans the five days before ovulation and ovulation day itself.

Because ovulation timing varies from cycle to cycle — and becomes unpredictable in perimenopause — these dates are estimates only. Do not rely on them to prevent or plan a pregnancy without talking to a clinician.