Fertile Window Calculator
How the six-day fertile window is calculated, why it beats guessing, and how to use it.
The fertile window is the five days before ovulation plus ovulation day, calculated as 14 days before the next expected period. For a 28-day cycle starting October 1: ovulation around October 15, fertile window October 10-15. The calculator needs only your last period date and average cycle length.
How the calculation works
Two inputs: the first day of your last period and your average cycle length. The calculator adds the cycle length to get the next expected period, subtracts 14 days for the estimated ovulation day, then marks the five days before ovulation through ovulation day as the fertile window.
Example, 28-day cycle starting October 1: next period October 29, ovulation October 15, fertile window October 10 through 15. Example, 35-day cycle starting October 1: next period November 5, ovulation October 22, fertile window October 17 through 22. The window slides with cycle length; only the 14-day luteal anchor stays fixed.
Why six days
The window length comes from biology, not convention: sperm survive up to five days in fertile cervical mucus, and the egg survives 12 to 24 hours after release. Five days before plus the day itself is the span in which intercourse can lead to conception.
Conception odds are not flat across the six days: they rise toward ovulation, peaking roughly two days before and the day before ovulation in most studies. Day -5 has low but nonzero odds; days -2 to 0 carry the bulk of conceptions.
Using the window for conception
Time intercourse for the two to three days before expected ovulation rather than ovulation day itself, so sperm are waiting when the egg releases. Every-other-day through the window performs about as well as daily in studies, with less pressure.
Because the calendar window is an estimate, many couples widen it by a day or two on each side or confirm with LH strips. If conception has not happened after 12 months of well-timed trying (6 months if over 35), guidelines recommend a fertility evaluation.
Limits of the calculator
The calculator assumes regular cycles and a textbook 14-day luteal phase. Irregular cycles, recent hormonal contraception changes, breastfeeding, stress, illness, and perimenopause all widen the error bars. Treat the output as the center of a window, not a precise appointment.
And the standing caution: fertile-window estimates are family-planning information, not contraception. Calendar-based avoidance has high typical-use failure rates because the window can shift without warning.
Skip the arithmetic
Calculate your fertile window now with the free ovulation calculator.
Fertile window questions
How do I calculate my fertile window?
Estimate ovulation as 14 days before your next expected period, then count the five days before ovulation plus ovulation day itself as the fertile window. The calculator does this from two inputs: your last period start date and your average cycle length.
How many days after my period am I fertile?
It depends on your cycle length, because the window is anchored to ovulation, which sits about 14 days before the next period. In a 28-day cycle the fertile window starts around cycle day 9; in a 35-day cycle, around day 17. Counting forward from the last period without adjusting for cycle length is the most common error.
Can the fertile window change each month?
Yes. Anything that shifts ovulation, stress, illness, travel, exercise changes, perimenopause, shifts the fertile window with it, since the window is defined relative to ovulation day. Calendar estimates are averages over your cycles; LH test strips and cervical mucus confirm the current month in real time.