Ovulation Calculator

Estimate ovulation day and fertile window from your last period and cycle length.

First day of last period

Cycle length (days)

Next ovulation

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Fertile window start

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Fertile window end

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Next period (est.)

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Cycle phase today

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Ovulation typically happens 14 days before the next period starts (not 14 days after the last one — important for non-28-day cycles). The fertile window covers 5 days before ovulation + the day of ovulation itself, because sperm can live up to 5 days inside the female reproductive tract.

How this works

For a typical 28-day cycle, ovulation lands around day 14. We compute next ovulation = LMP + cycle length − 14. The fertile window is the 5 days before ovulation plus the ovulation day itself — 6 days total. This is an estimate; for trying-to-conceive use cases, ovulation predictor kits or basal body temperature tracking are more accurate.