Fertile Days Calculator

The fertile window is the span of days within each menstrual cycle during which conception is biologically possible. It is wider than most people expect: because sperm can survive in the female reproductive tract for up to five days in fertile cervical mucus, the window opens several days before ovulation itself. This calculator estimates your fertile window, peak ovulation day, and how many days remain until estimated ovulation — all based on your last period start date and average cycle length. Ovulation timing is estimated using the luteal phase constant: ovulation typically occurs approximately 14 days before the next expected period, which means the ovulation cycle day equals your cycle length minus 14. For a 28-day cycle that is cycle day 14; for a 35-day cycle it is cycle day 21. The fertile window in this calculator spans from 5 days before estimated ovulation through 1 day after. This calculator is for informational awareness of general cycle patterns. It is not a family planning tool, not a contraception method, and should not be relied upon for preventing or achieving pregnancy without additional guidance from a healthcare provider. Individual ovulation timing varies considerably from these averages even in people with generally regular cycles.

Ovulation is estimated by subtracting 14 days from your cycle length (the luteal phase constant) to get the ovulation cycle day, then adding that many days to your last period start date to get the calendar date. The fertile window opens 5 days before ovulation — because sperm can survive in fertile cervical mucus for up to five days — and closes 1 day after ovulation, when the egg is no longer viable. Days until ovulation is calculated from today's date. All results are estimates based on average cycle patterns and the luteal phase constant. Actual ovulation timing varies between individuals and cycles, especially in people with irregular or varying cycle lengths.