Enter your due date or last period date to get an estimated conception date, based on standard obstetric timing.
From a due date: conception ≈ due date − 266 days. From the first day of your last period: conception ≈ LMP + 14 days. These look unrelated but are not — the standard due-date convention itself is LMP + 280 days, and 280 − 266 = 14 exactly. Feed a due date that came from that same 280-day convention into the first formula and a last-period date into the second, and they land on the identical day, because both are just "ovulation, roughly two weeks into the cycle" viewed from opposite ends of the pregnancy.
Worked check: an LMP of 1 January 2026 gives a due date of 8 October 2026 (adding 280 days) and a conception estimate of 15 January 2026 (adding 14). Run 8 October back through the due-date formula — subtract 266 days — and it returns the same 15 January 2026.
The two only diverge when the due date on hand did not come from that plain 280-day rule — for instance, one adjusted for a longer or shorter cycle, or set by an ultrasound measurement instead. In that case the two modes are genuinely estimating from different evidence, and can reasonably disagree by several days.
The calculator returns one day because a single number is what a form field needs, but the underlying biology does not commit to one day. An egg is viable for roughly 12–24 hours after ovulation, while sperm can survive up to about five days waiting for it — so conception can only occur inside that stretch, but ovulation itself does not land on the same cycle day every month even for someone with a textbook-regular cycle. Treat the result as the center of a roughly week-wide range, not a precise event, and further out for anyone whose cycle is not close to 28 days.
Legal paternity and custody timing questions that hinge on a conception date run into the same uncertainty, which is why they are typically handled as a range bounded by gestational-age limits at birth rather than a single computed day — genetic testing settles parentage directly and makes the timing question moot in practice, but the underlying date estimate is exactly this arithmetic, with exactly this margin of error.
This tool provides a general estimate for personal reference, not a medical, legal or forensic determination. For due-date planning see the pregnancy due date calculator, for week-by-week tracking the pregnancy week calculator, and for timing a pregnancy attempt the ovulation calculator.
From a known due date, conception is estimated as due date minus 266 days. From a known last menstrual period (LMP), conception is estimated as LMP plus 14 days, since ovulation and conception typically occur around two weeks into a standard 28-day cycle. Both routes are offered here since people commonly know one date but not the other.
Due date minus 40 weeks (280 days) gets you back to the first day of the last menstrual period, not conception itself — that's the calculation the Pregnancy Due Date Calculator uses in reverse. Conception happens roughly two weeks after LMP, so the conception estimate here uses 266 days instead of 280 to land on the more likely actual date of conception rather than the start of that cycle.
It's an estimate based on average cycle timing, not a precise measurement — actual ovulation and conception timing varies by individual cycle length, and can shift the real date by several days in either direction, especially for people with irregular cycles. Treat the result as a reasonable window to think about, not an exact date. See the Ovulation Calculator if you're trying to estimate a fertile window going forward rather than looking back.
This gives a general estimate useful for personal timeline and planning purposes, but it is not precise enough for legal or paternity determinations, which rely on DNA testing rather than date-based estimation. Use this tool for general context only, not as evidence for any formal timing question.
Worked example: a due date of January 15, 2027 implies an estimated conception date of January 15, 2027 minus 266 days ≈ April 24, 2026. A last period date of June 1, 2026 implies an estimated conception date of June 1, 2026 plus 14 days = June 15, 2026.
Trying to conceive going forward? See the Ovulation Calculator. Already expecting? See the Pregnancy Due Date Calculator. For cycle forecasting, the Period Calculator projects the next six periods with ovulation windows.