Ovulation Calculator

How the Ovulation Calculator Works

Ovulation — when an ovary releases an egg — happens about 14 days before your next period starts, because the second half of the cycle (the luteal phase) is fairly constant. The calculator takes the first day of your last period, adds your cycle length, and steps back 14 days to estimate the ovulation date.

The fertile window spans the five days before ovulation through the day of ovulation itself, since sperm can survive several days while the egg lasts about a day. The two days leading up to ovulation are the most fertile. If you’re trying to conceive, timing intercourse in this window gives the best chance; if you’re avoiding pregnancy, note that cycle timing is only a rough guide and not a reliable contraceptive method.

ovulation ≈ last period + (cycle length − 14 days)

Note: these are estimates based on standard averages. Every pregnancy is different — your doctor or midwife, using an ultrasound, gives the most accurate dates.