Conception Calculator

About the Conception Calculator

Conception occurs when a sperm fertilizes an egg during or shortly after ovulation. For a standard 28-day cycle, ovulation happens around day 14 — meaning the most likely conception date is approximately 14 days after the start of your last menstrual period (LMP). If you know your due date, you can work backward: subtract 266 days (38 weeks) to arrive at the estimated conception date. For a broader view of your cycle and upcoming fertile periods, try our Ovulation Calculator, which also shows the next three months of ovulation dates.

The fertile window spans 6 days: the 5 days leading up to ovulation, plus the day of ovulation itself. This is because sperm can survive in the female reproductive tract for up to 5 days, while the egg is viable for only 12–24 hours after release. The 2–3 days immediately before ovulation are generally considered peak fertility. Because of this biological window, there is inherent uncertainty in pinpointing an exact conception date — which is why this calculator always provides a range rather than a single day. To estimate your due date from the conception date or LMP, visit our Due Date Calculator.

It is important to remember that these calculations assume a regular 28-day cycle. Women with shorter or longer cycles will have different ovulation timing, and the actual day of ovulation can vary even within the same person from month to month. Factors such as stress, illness, and hormonal changes can all affect cycle length. Ovulation predictor kits (OPKs), basal body temperature (BBT) charting, and cervical mucus monitoring are practical tools for more accurately identifying ovulation. Our Pregnancy Calculator can help you track milestones once pregnancy is confirmed.