Conception Date Calculator

How the Conception Date Calculator Works

Conception happens at ovulation, roughly two weeks after the first day of your last period. Working from a due date, the calculator subtracts 266 days — the length of pregnancy measured from conception — to estimate when it occurred. Working from your last period, it adds your cycle length minus 14 days to land on the likely ovulation, and therefore conception, date.

Because the exact timing of ovulation and fertilization varies, conception is best thought of as a window of a few days rather than a single date. This tool shows that window along with the matching last-period and due dates so the figures can be cross-checked.

conception ≈ due date − 266 days

Note: these are estimates based on standard averages. Every pregnancy is different — your doctor or midwife, using an ultrasound, gives the most accurate dates.