Conception Date Calculator

How conception date is estimated

There are two ways to estimate when conception occurred. If you know your due date, the calculator subtracts 266 days (280 days of full gestation minus the 14-day gap between the LMP and ovulation). If you know your LMP instead, the calculator adds 14 days to reach the estimated ovulation and fertilisation date. Either approach assumes an average 28-day cycle with ovulation at day 14.

Conception dates are estimates, not certainties: ovulation timing varies between individuals and cycles, and fertilisation can occur over a window of hours to days. All calculation happens locally in your browser.

FAQ

Why 266 days rather than 280? 280 days is counted from the LMP; conception happens roughly 14 days later, so 280 - 14 = 266 days before the due date.

Can I use this to determine paternity? No. Conception estimates span multiple days and should never be used for legal or paternity conclusions.

My cycle is not 28 days — how does that affect the result? A longer cycle means ovulation happens later than day 14, so the estimate may be off by the same number of extra days. For irregular cycles, the margin of error is wider.

Not medical advice. This tool provides estimates only. Always consult a qualified healthcare provider for clinical decisions.