Ovulation & Fertile Window Calculator

How it works

Ovulation = last period + (cycle length − luteal phase) days, i.e. cycle day (cycle − luteal + 1) · luteal method: fertile window = ovulation − 5 days to ovulation + 1 day · calendar method: cycle day (shortest − 18) to day (longest − 11) · Standard Days: cycle days 8–19 (26–32 day cycles) · LH tests from ≈ 4 days before ovulation

Ovulation is timed backward from the next period, not forward from the last, because the luteal phase (ovulation to menstruation) is fairly constant at about 14 days while the follicular phase varies. Sperm can survive up to about five days in the reproductive tract and the egg lives roughly a day, so the luteal-phase method takes the five days before ovulation through the day after as the fertile window. If your cycles vary, the calendar method uses your shortest and longest recent cycles instead (shortest − 18 to longest − 11), and the Standard Days method treats days 8–19 as fertile for cycles of 26–32 days. The day-by-day table labels every day of the cycle, the date checker tells you where any date falls, and the start date for ovulation tests helps you confirm the estimate. Cycle length and ovulation shift month to month, so treat these dates as estimates.

Worked example

With a 28-day cycle, a 14-day luteal phase and a last period starting 1 June 2026, the next period is due 29 June and ovulation is estimated 14 days earlier, on 15 June (cycle day 15). The fertile window runs 10–16 June, peaking 14–15 June, and ovulation tests are worth starting on 11 June. With the calendar method and cycles of 26–31 days, the window is day 8 to day 20: 8–20 June.

Sources: Wilcox, Weinberg & Baird (1995) — Timing of sexual intercourse in relation to ovulation, NEJM · ACOG — Fertility awareness-based methods of family planning · CDC — Contraception: how effective are birth control methods?

Frequently asked questions