How to find your fertile window
Step by step
- Note the first day of your period.
- Count your usual cycle length (first day of one period to the first day of the next).
- Ovulation ≈ next period − 14 days.
- Fertile window = the 5 days before ovulation and ovulation day.
Example: a 28-day cycle starting 1 October gives ovulation around 15 October and a fertile window from 10 to 15 October.
Longer or shorter cycles
Cycles from 21 to 35 days are normal (NHS). Example: a 32-day cycle starting 1 October gives a next period around 2 November, so ovulation around 19 October and a fertile window from 14 to 19 October.
Why 6 days
Sperm can live up to 7 days inside the body (NHS), so sex in the days before ovulation can lead to pregnancy, not only sex on ovulation day. Use the ovulation calculator for your own dates.
Sources
- NHS: Periods and fertility in the menstrual cycle · checked October 26, 2026
- NHS: Trying to get pregnant · checked October 26, 2026
- NHS: Natural family planning (fertility awareness) · checked October 26, 2026
- CDC: Infertility FAQs · checked October 26, 2026