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Ovulation calculator

Ovulation and fertile days calculator

Most fertile day (ovulation) —
Fertile window
—
Estimated next period
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Estimates ovulation ~14 days before your next period and the fertile window (5 days before and 1 day after). Approximate and less reliable with irregular cycles; not a method of contraception.

This ovulation calculator estimates your most fertile day, your fertile window and your next expected period from the first day of your last period and your typical cycle length.

How the estimate works

Enter the first day of your last period and your cycle length in days (28 by default), and the result updates instantly: the estimated ovulation day, the fertile window around it, and your next expected period. This is orientative — it is not a method of contraception and becomes less accurate with irregular cycles.

FAQ

How is ovulation day estimated?
It's calculated as roughly 14 days before your next expected period, based on the first day of your last period and your average cycle length.
What is the fertile window?
The fertile window shown here spans 5 days before ovulation to 1 day after — the days around ovulation when pregnancy is most likely, since sperm can survive several days and the egg for about a day.
How reliable is this for irregular cycles?
Less reliable. This calculator assumes a fairly regular cycle length; if your cycles vary a lot from month to month, the estimated ovulation day and fertile window are only a rough guide, not a precise prediction.