Ovulation Calculator

Ovulation is counted backward from the next period, not forward from the last one. The stretch after ovulation, the luteal phase, tends to stay near two weeks, while the stretch before it flexes. So a 35-day cycle ovulates much later than a 28-day cycle, around day 21 instead of day 14.

The fertile window opens about five days earlier because sperm can survive that long. Cycle length shifts the day: with a 21 day cycle ovulation lands near day 7, with 28 days near day 14 and with 35 days near day 21, since the luteal phase stays close to 14 days. That phase usually runs 12 to 16 days, so an average of several cycles beats a single one.

Use your own average cycle length from several months, not the 28-day default. One long or short cycle can move the estimate by a week.

See the due date and conception date calculators for pregnancy timing, and the period calculator for upcoming cycles. The pregnancy week page tracks progress after conception. A healthcare provider can advise on fertility concerns and preconception care.