Ovulation calculator

When does ovulation happen?

Ovulation usually happens about 14 days before your next period, because the second half of the cycle (the luteal phase) is fairly constant in length. With a 28-day cycle that is day 14; with a 35-day cycle it is day 21.

The fertile window

Sperm can survive up to about five days, while an egg lives about one day. A landmark 1995 study in the New England Journal of Medicine found pregnancies came almost only from intercourse during the six days ending on ovulation day, with the best chances in the two to three days before ovulation.

Worked example

If your last period started on March 1 and your cycle is 28 days, ovulation is estimated on March 15, your fertile window runs from March 10 to March 16, and your next period is due around March 29.

Improve the estimate

Calendar methods are rough: cycles shift from month to month. Ovulation predictor kits (LH tests), basal body temperature charts and cervical mucus changes help pinpoint it. If you have irregular cycles or have been trying for a year (six months if over 35), talk to a doctor.

This calculator is not a method of birth control.

The page subtracts 14 days from your next expected period, which is the first day of your last period plus your cycle length. The fertile window runs from five days before the estimated ovulation to one day after. The next two ovulation dates repeat the same cycle length. Everything runs in your browser; nothing is stored or sent.