Ovulation and Fertile Window Calculator

Estimate ovulation day and fertile window from your last period and cycle length. Privacy-first: no accounts, no tracking, no data leaves your device.

Your cycle

First day of your last period (LMP)

Cycle length (days)

The number of days from one period start to the next.

Advanced: luteal-phase length

The luteal phase is the time from ovulation to the next period. The default is 14 days; if you've charted your cycles and know your own luteal length, you can override it (9–17 days).

Luteal length (days, optional)

Export

How we choose and check our formulas