Ovulation Calculator

Enter your last period date and cycle length to find your fertile days.

First day of last period (YYYY-MM-DD)

Average cycle length (days)

Estimated ovulation date —

Fertile window —

Next period expected —

Ovulation typically occurs 14 days before the next period. The fertile window is approximately 5 days before ovulation through 1 day after. Sperm can survive up to 5 days; the egg survives 12–24 hours.

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How to Calculate Your Fertile Window

Ovulation typically occurs about 14 days before your next expected period. For a 28-day cycle, that is around day 14. For a 32-day cycle, around day 18.

The fertile window spans approximately 6 days: the 5 days before ovulation plus ovulation day itself. This is because sperm can survive in the reproductive tract for up to 5 days.

Ovulation typically occurs 12–16 days before the next expected period — approximately day 14 in a 28-day cycle, but day 19 in a 33-day cycle and day 11 in a 25-day cycle. The fertile window spans from 5 days before ovulation (sperm survival) through the day of ovulation itself — a 6-day window each cycle.

Basal body temperature (BBT) method: measure temperature before getting out of bed each morning. Temperature rises 0.2–0.5°C (0.4–1.0°F) after ovulation due to progesterone. This rise confirms ovulation has occurred but cannot reliably predict it in advance — the rise is a retrospective signal.

Luteinizing hormone (LH) surge: home ovulation predictor kits (OPKs) detect the LH surge that triggers ovulation 24–36 hours later. A positive OPK indicates the fertile window is open; intercourse within 12–36 hours of a positive OPK gives the highest conception probability per cycle.

Cycle variability: many people have cycles that vary 4–7 days month-to-month. Apps and calculators average across cycles you report; the first 1–3 months of tracking are less accurate as baseline cycle length is established. Irregular cycles (PCOS, perimenopause, recent hormonal contraceptive use) require more monitoring with OPKs or ultrasound for reliable timing.

All estimates are based on average cycle data. Consult your healthcare provider for fertility guidance.