How the fertile window is found
Ovulation happens about 14 days before your next period, so it is counted back from the end of the cycle: last period + cycle length − 14. Sperm can survive up to five days, and the egg about one, which gives a six-day fertile window ending the day after ovulation. The two days before ovulation and the day itself are the most fertile.
Common questions
Why a six-day fertile window, not just the ovulation day?
Sperm can survive up to five days in the body waiting for an egg, while the egg itself is viable for about a day after release — so the window covers the five days before ovulation plus the day itself.
What if my cycle length varies month to month?
Enter your typical or average length — the calculator projects forward assuming that length repeats, so the further out a predicted cycle is, the less reliable it gets if your cycles are irregular.
An estimate for planning only, not a contraceptive method. See a doctor if you have been trying for a year (six months if over 35).