Fertility Window Calculator
Your fertile window is the handful of days each cycle when conception is possible. Enter your last period and cycle length to see the window, the peak days and the ovulation day.
Your numbers
Result
- Ovulation day
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- Peak fertility
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How the fertile window is worked out
Ovulation lands about 14 days before the next period, so on a cycle length − 14 day. The fertile window spans the 5 days before ovulation plus the day after — six days in total. That is because sperm can survive up to five days waiting for the egg, which itself lives about 24 hours.
Peak days
Not all fertile days are equal. The chance of conceiving is highest in the two or three days leading up to and including ovulation. If you are trying to conceive, timing intercourse to those peak days — or every one to two days across the whole window — gives the best odds.
When the calendar is less reliable
This assumes a regular cycle. If yours varies, the window shifts and the dates are only a guide. Ovulation predictor kits (which detect the LH surge), tracking cervical mucus, and basal body temperature all pin the window down more precisely than the calendar alone.
Informational and educational result. Not a substitute for professional advice.
Frequently asked questions
Sources
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