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Free tool · no sign-upEnter your last period date to find your estimated ovulation day and fertile window — to help you plan.
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Ovulation is the release of a mature egg from the ovary, and it usually happens about 14 days before your next period. The fertile window is the few days when the chance of conceiving is highest: because sperm can live up to 5 days, the window covers the 5 days before ovulation and the ovulation day itself — six days in total, ending on ovulation day.
The calculation subtracts 14 days (the luteal phase) from your cycle length to find the estimated ovulation day, and shows the fertile window as the six days ending on it. This is an estimate — ovulation timing can vary month to month, so tracking for a few cycles clarifies the pattern.
Ovulation usually happens about 14 days before your next period. Subtracting 14 from your cycle length gives the estimated ovulation day. It is an estimate; the real timing can vary month to month.
Because sperm can survive up to 5 days and the egg is fertile for about 1 day, the fertile window spans the 5 days before ovulation plus the ovulation day itself. The highest chance of conceiving is the 1–2 days just before ovulation.
It gives a good estimate for regular cycles, but ovulation can shift with stress, sleep and health. For more precise tracking, pair it with basal body temperature or ovulation tests.
With irregular cycles a single estimate is less reliable. Logging your cycle for a few months clarifies the pattern; if irregularity persists, it is worth talking to your doctor.
This calculation is based on an average 14-day luteal phase and on FIGO / ACOG fertility criteria. Results are estimates and don’t replace medical advice. If you have questions about conceiving, or a situation that has continued for a while, talk to your doctor.
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