Conception Date Estimate Calculator

About this calculator

Estimating a likely conception date works backward from ovulation timing within the menstrual cycle, since conception can only occur around ovulation — typically about 14 days before the next period starts, based on the luteal phase length staying relatively fixed even when the follicular phase (and therefore total cycle length) varies. Given a cycle length, the calculation locates the estimated ovulation day and the surrounding fertile window (a few days before ovulation through the day of ovulation, since sperm can survive several days awaiting the egg), and identifies the most likely conception day within that window.

People trying to conceive, and those trying to reconstruct a pregnancy timeline, use this kind of cycle-based estimate to identify the fertile window for family planning, or to cross-check an estimated due date against a known last-menstrual-period date. It's a statistical estimate based on typical cycle physiology, not a measurement of any specific event, since actual ovulation timing varies cycle to cycle.

This calculator takes your cycle length and the start date of your last period, and returns the estimated conception day and the start of your fertile window.