How ovulation is predicted

Ovulation — when an ovary releases an egg — usually happens about 14 days before your next period starts, regardless of how long your cycle is. So for a 28-day cycle ovulation is around day 14, while for a 32-day cycle it’s closer to day 18.

This calculator estimates your next period from your cycle length, then counts back 14 days to predict ovulation, and marks the fertile window around it.

Your fertile window

The egg lives for only about 12–24 hours, but sperm can survive in the body for up to five days. That creates a fertile window of roughly six days: the five days before ovulation plus ovulation day itself. The two days just before ovulation are the most fertile.

If you’re trying to conceive, having sex every one to two days through the fertile window gives the best chance. If you’re avoiding pregnancy, remember that calendar prediction alone is not a reliable contraceptive — see below.

How reliable is it?

Calendar-based prediction works best if your cycles are regular. Stress, illness, travel and conditions such as PCOS can shift ovulation, so the predicted day can be off by several days. Tracking basal body temperature or using ovulation predictor kits, which detect the hormone surge before ovulation, gives a more precise picture.

Because of this variability, the rhythm method is one of the least reliable forms of contraception. If you are trying to avoid pregnancy, speak to your doctor about more dependable options. If you’re trying to conceive and haven’t succeeded after 6–12 months, your OB-GYN can help.

Frequently asked questions

Sources & references