BYTETOOLS

Conception Date Calculator

Work back from a due date, last period or ultrasound to the likely conception window, with cycle length taken into account and a full timeline.

Conception window

Pick a date above to work back to a conception window.

What is the Conception Date Calculator?

Conception is estimated at ovulation, which falls on cycle length minus the luteal phase — day 14 of a 28-day cycle. From a due date that works out at roughly 266 days earlier, and because sperm survive several days the honest answer is a six-day window rather than one date.

  • Three starting points: due date, last menstrual period, or ultrasound gestational age
  • A six-day fertile window rather than a single false-precision date
  • Cycle length and luteal phase both configurable, not hardcoded to 28 and 14
  • Timeline table showing how every date is derived
  • Likely implantation window at 6–12 days after conception
  • Current gestational age in weeks and days, on the clinical convention

How to use the Conception Date Calculator

  1. 1

    Choose whether to work back from a due date, a last menstrual period, or an ultrasound gestational age.

  2. 2

    Enter that date. For the ultrasound option, also enter the gestational age in weeks and days that the scan reported.

  3. 3

    Set your average cycle length and, if you know it from charting, your luteal phase — otherwise leave it at 14 days.

  4. 4

    Read the conception window, then check the timeline table for the implied last period, implantation window and due date.

  5. 5

    Copy the dates if you want to keep them.

About the Conception Date Calculator

The ByteTools Conception Date Calculator works backwards from whatever date you already have. Start from an estimated due date, from the first day of your last period, or from a gestational age given at an ultrasound, and it reconstructs the rest of the timeline.

It applies Naegele's rule — 280 days from the last period — adjusted for your cycle length, and places ovulation at cycle length minus your luteal phase rather than assuming everyone ovulates on day 14. The answer is deliberately given as a six-day fertile window ending on the estimated ovulation day, because sperm can survive for several days and a single date would be false precision. The timeline table also shows the likely implantation window and how far along the pregnancy is today.

Everything is calculated in your browser and none of the dates you type are uploaded or stored. These are calendar estimates only, not medical advice and not proof of anything.

Frequently asked questions

How do I calculate conception date from a due date?

Subtract 266 days from the due date for a standard 28-day cycle — that is 280 days of gestation minus the 14 days between the last period and ovulation. If your cycles are longer or shorter than 28 days, the offset shifts by the same number of days, which this calculator handles for you.

How accurate is a conception date calculator?

It is a calendar estimate, not a measurement. Ovulation genuinely varies by several days from cycle to cycle even in women with regular periods, so the true conception date can sit either side of the window shown. A first-trimester ultrasound dates a pregnancy far more accurately than any arithmetic on a remembered period.

Why does the calculator give a window instead of a date?

Because sperm can survive several days in fertile cervical mucus while an egg is only viable for about 12 to 24 hours. That means intercourse in the five days before ovulation, or on the day itself, can result in conception — so a six-day window is the honest answer.

Is conception date the same as gestational age?

No, and the difference trips people up constantly. Clinicians count gestational age from the first day of your last period, about two weeks before conception, so a pregnancy that is '8 weeks' by that convention is only about 6 weeks since conception.

Can this tell me who the father is?

No. It gives a probabilistic window from arithmetic, with several days of genuine uncertainty either side, and it cannot establish paternity. Only DNA testing can do that, and any medical or legal question about a pregnancy should go to a clinician or lawyer.

Does IVF change the calculation?

Yes — with IVF the conception date is known exactly, so working backwards is unnecessary. Due dates for IVF pregnancies are counted from the transfer date adjusted for embryo age, which is a different calculation from Naegele's rule.

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