Ovulation Calculator
Estimate your ovulation day and fertile window from your last period and typical cycle length.
The cycle math
Fertile window = ovulation day − 5 through ovulation day
28-day cycle: ovulation around day 14, fertile window days 9–14, most fertile days 12–13.
32-day cycle: ovulation shifts to about day 18 — the luteal phase (ovulation to period) stays ~14 days, so longer cycles ovulate later.
Reading the fertile window
Because sperm can survive up to five days and the egg only about a day, the fertile window opens several days before ovulation and closes shortly after — the two days before ovulation are peak. Calendar prediction works best with regular cycles; if yours vary, ovulation predictor kits, basal body temperature charts, and cervical mucus changes give real-time confirmation this estimate can't. Pair with the due date calculator once pregnancy is confirmed.
Frequently asked questions
When do I ovulate?
About 14 days before your next period — day 14 in a 28-day cycle, day 18 in a 32-day cycle.
What is the fertile window?
The ~6 days ending on ovulation; the two days before are most fertile.
How accurate is calendar prediction?
A cycle-average estimate — less reliable if irregular. Kits and BBT tracking are more precise.
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Medical disclaimer: A calendar-based estimate, not medical advice or a contraceptive method. For fertility concerns or reliable family planning, consult a healthcare provider. Last reviewed: July 2026.