Ovulation calculator
Estimate the probable date of your ovulation, your fertility window, and the date of your next period, from the first day of your last period and your average cycle length. Calculation entirely local, no health data is transmitted.
How it works
For most people with a regular menstrual cycle, ovulation occurs about 14 days before the start of the next period (not 14 days after the start of the last period, except for an exact 28-day cycle). This tool therefore subtracts 14 days from your total cycle length to pinpoint the ovulation day, then derives a fertility window of a few days around that date.
Why does the fertility window start before ovulation day?
Sperm can survive several days in the reproductive tract, while the egg is only fertilizable for about 24 hours after ovulation. The fertility window therefore generally includes the 5 days before ovulation as well as the following day, to cover the entire period during which a pregnancy remains possible.
Is this estimate reliable for all cycles?
It is mainly indicative for regular cycles. In the case of irregular cycles, polycystic ovary syndrome, or other hormonal particularities, the actual ovulation date can vary significantly from one month to the next. An ovulation test or basal temperature tracking, or even medical advice, is recommended for a more precise estimate.
This information stays strictly private: no cycle data is sent or kept outside your browser.