Ovulation Calculator

How ovulation is estimated

A menstrual cycle has two phases. The follicular phase, before ovulation, varies in length from person to person. The luteal phase, after ovulation, is more consistent at about 12–16 days, typically 14. So ovulation usually happens about 14 days before the next period, not 14 days after the last one.

Ovulation day ≈ First day of last period + (Cycle length − Luteal phase)

The fertile window

Sperm can survive in the reproductive tract for up to five days, while an egg lives for 12–24 hours after release. The fertile window is therefore about six days: the five days before ovulation and the day of ovulation itself. The two or three days just before ovulation carry the highest chance of conception.

Making the estimate more accurate

• Ovulation predictor kits detect the surge in luteinizing hormone (LH) 24–36 hours before ovulation.
• Basal body temperature rises by about 0.3–0.5 °C (0.5–1 °F) after ovulation, confirming it has happened.
• Cervical mucus becomes clear and stretchy, like egg white, around the most fertile days.
• Tracking several cycles gives a better average cycle length than a single month.

Calendar methods are estimates and are not reliable for preventing pregnancy. If you have very irregular cycles or have been trying to conceive for over a year (six months if over 35), talk to a doctor.