Conception Window Calculator
Pregnancy is only possible during a narrow window each cycle — approximately 6 days ending on ovulation day. Enter your last period date and cycle length to see your fertile window highlighted on a full cycle calendar, with your peak fertility days and estimated ovulation date.
Enter your cycle details
Use the first day of your most recent period — the day bleeding began, not spotting.
Day 1 = the first day of actual bleeding. If your period started today, select today's date.
Count from the first day of one period to the first day of the next. Most women range from 21–35 days — the average is 28. Enter your personal average for the most accurate window.
The luteal phase is the time from ovulation to the next period. It is generally consistent for each person — tracking a few cycles in an app like Clue or Natural Cycles can give you your exact number.
The science of your fertile window
Each month, one of your ovaries releases a mature egg — a process called ovulation. The egg is viable for just 12–24 hours. However, because sperm can survive 3–5 days in the female reproductive tract, the fertile window extends 5 days before ovulation plus ovulation day itself — six days total.
Ovulation doesn't happen on a fixed calendar date; it happens a consistent number of days before your next period. This is the luteal phase — the time from ovulation to menstruation — which is typically 14 days and relatively stable. This is why the fertile window is calculated by counting backwards from when you expect your next period, not forwards from your last one.
For women with irregular cycles, the fertile window is harder to predict by date alone — this is where physical signs (cervical mucus, basal body temperature, OPK tests) become especially valuable.