The results are for information only and do not replace the advice, diagnosis or opinion of a doctor or other qualified health professional.

How the ovulation calculator works

Ovulation does not happen on a fixed day of the cycle. It happens about one luteal phase before the next period begins, and the luteal phase usually lasts 12 to 16 days, with 14 as the average. That is why this calculator asks for three things: the first day of your last period, the usual length of your cycle in days, and the length of your luteal phase. The estimated ovulation day is the first day of the last period plus the cycle length minus the luteal phase. If you do not know your luteal phase, leave the default of 14.

An egg survives for roughly one day, but sperm can live up to five days in the reproductive tract, so the fertile window spans the five days before ovulation plus the day of ovulation itself: six days in total. The chance of conception is not equal on all of them. In the study by Wilcox and colleagues (New England Journal of Medicine, 1995), intercourse on each day from five days before to the day of ovulation was followed by a pregnancy in 10%, 16%, 14%, 27%, 31% and 33% of cycles. The last three days are therefore the most fertile, and the table and chart show these figures day by day.

Example: the last period began on October 1, 2026, the cycle is 30 days long and the luteal phase is 14 days. Ovulation falls on October 1 plus 30 minus 14, which is 16 days later: October 17, 2026, on cycle day 17. The fertile window runs from October 12 to October 17, the most fertile days are October 15 to 17, and the next period is expected on October 31, 2026, which is also the first day a home pregnancy test is reliable. The calculator repeats the same pattern for the following five cycles.

These dates are estimates based on averages. Cycles vary from month to month, and stress, illness, travel and breastfeeding can shift ovulation, so the real day may differ by several days. To pinpoint it, combine the calculator with ovulation (LH) test strips, basal body temperature charts or observation of cervical mucus. This tool is not a method of contraception and must not be used to avoid pregnancy. If you have been trying to conceive for 12 months, or for 6 months when you are over 35, talk to a doctor.