Conception Calculator

Estimate the date of conception from a due date, last period, or birth date

Estimated due date

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Estimated conception date

January 15, 2026

Conception window: Jan 10 – Jan 16

Reference due date

October 8, 2026

How conception is estimated

Conception ≈ due date − 266 days (≈ LMP + 14 days)

Conception is estimated as ovulation, about 266 days before a full-term birth or 14 days after the last period. From a known birth date the same offset is applied. Actual timing varies.

What is a conception calculator?

A conception calculator estimates the day a pregnancy most likely began. Because the exact moment of conception is rarely observed, it is worked out from a date you do know: an estimated due date, the first day of the last menstrual period (LMP), or — for a baby already born — the date of birth. Conception happens at ovulation, about two weeks after the LMP and roughly 266 days before the due date. The tool also shows a conception window, since sperm can fertilise an egg over a span of several days rather than a single instant.

How to use it

1. Choose what you know: the estimated due date, the first day of your last period, or the baby's birth date. 2. Enter that date. The calculator estimates the date of conception and a short conception window around it, and shows the due date it is anchored to. If you used a known birth date, the conception estimate is back-dated from the actual birth rather than a predicted due date.

How it is calculated

Conception is estimated as ovulation, which falls about 266 days before a full-term birth and about 14 days after the LMP: Conception ≈ due date − 266 days Conception ≈ LMP + 14 days When the due date is the standard LMP + 280 days, both routes agree. For an LMP of January 1, 2026 the due date is October 8, 2026 and conception falls around January 15, 2026. From a known birth date, the same 266-day offset is applied to the actual day of birth.

Reading your results

The conception date is an estimate, not an exact event — the window of a few days reflects how long sperm survive and the uncertainty in ovulation timing. It is useful for understanding roughly when a pregnancy began, satisfying curiosity, or framing early development, but it should not be used to assign paternity or make legal or medical decisions. An early ultrasound, which measures the embryo directly, gives a more accurate dating of the pregnancy.

Frequently asked questions

How is conception date calculated from a due date? ▾

Conception is estimated about 266 days before the due date, since a full-term pregnancy lasts roughly 38 weeks from conception (40 weeks from the last period). The tool subtracts 266 days from the due date you enter.

Conception date vs. LMP — what's the difference? ▾

Gestational age is counted from the last menstrual period, about two weeks before conception. So conception is roughly LMP + 14 days, and a pregnancy is about two weeks 'pregnant' on paper before conception actually occurs.

Can this tell me exactly when I conceived? ▾

No. It gives a likely date and a short window. Because ovulation timing varies and sperm can survive several days, the exact day cannot be pinned down from dates alone.

Should I use this to determine paternity? ▾

No. A calendar estimate is not reliable enough for paternity or legal questions. Those require medical testing and professional advice.

This conception calculator provides a general estimate using the LMP / Naegele method and is for informational purposes only — it is not medical advice and cannot establish paternity. Conception timing varies between individuals. Consult your doctor or midwife for accurate pregnancy dating.