Conception Date Calculator

Conception is a window, not a moment

People come to a tool like this expecting a date, and a date is what it gives you. It is worth understanding, before you write it anywhere, that the date is the middle of a range rather than a fact.

Here is the biology that makes it a range. An egg survives only about 12 to 24 hours after it is released, so fertilisation, if it happens, happens within roughly a day of ovulation. That part is tight, and it is why placing ovulation is the same job as placing conception. What is not tight is everything around it: ovulation itself shifts by a few days from cycle to cycle even in regular cycles, and sperm can survive up to five days waiting. Outside of IVF, where the moment is known exactly, a precise conception date genuinely cannot be pinned down.

So treat the output as a best estimate with a few days of slack on either side. Below is how it gets there, why the date sperm arrived is a different question from the date conception happened, and one thing about the due date shown next to it that is worth knowing.

How it places your date

Unlike a lot of conception calculators, this one asks for more than your last period, and that is the useful part. It wants your cycle length and your luteal phase, because ovulation is anchored to your next period rather than your last one.

It finds when your next period was due, steps back across your luteal phase to land on ovulation, and calls that conception:

Conception = (first day of last period + cycle length) − luteal phase

So with a last period starting 5 January 2026, a 28-day cycle and a 14-day luteal phase, ovulation lands on 19 January, and that is your estimated conception date. Change the cycle to 35 days and it moves to 26 January, a full week later, which is exactly the adjustment that calculators working from your last period alone cannot make.

If you do not know your luteal phase, 14 is the standard assumption and the tool's default. Most people never find out theirs, and 14 is a reasonable stand-in.

The night you conceived was probably not the night you think

This surprises nearly everyone, and it is the single most useful thing on this page.

Conception is not the same event as sex. Sperm can survive in the reproductive tract for up to five days, sitting in fertile cervical mucus, waiting for an egg that has not been released yet. So you could have had sex on a Friday and conceived on the following Tuesday. Research on this is unusually solid: nearly all pregnancies trace to sex in the six days ending on the day of ovulation, with the odds climbing to their highest on the day of ovulation itself, and the day after contributing almost nothing.

Which means the date this tool gives you is the estimated date of fertilisation, not a date on your calendar when something happened. The two can be nearly a week apart, both perfectly ordinarily. If you are trying to reconstruct events, that gap is the thing to hold on to.

About the due date it shows beside it

The tool also reports an estimated due date, and it is worth knowing which of the two conventions it uses, because they do not always agree.

There are two ways to date a pregnancy. The traditional one, Naegele's rule, adds 280 days to your last period. The other adds 266 days to conception. They give the same answer only when ovulation happened on day 14, which is precisely the assumption the first one quietly makes.

This tool uses the second. It takes the ovulation date it worked out from the cycle length and luteal phase you entered, and counts 266 days from there. So the conception date and the due date on your screen are built from the same numbers and describe the same pregnancy.

That matters most if your cycle is not 28 days. On a 35-day cycle you ovulate around day 21 rather than day 14, so the flat 280-day rule would put your due date roughly a week earlier than your own conception date implies. Counting from ovulation avoids that, and it is the more accurate method for anyone whose cycle is not the textbook length, which is most people. If your cycle is 28 days, both conventions give the same answer anyway.

None of which matters much once you have had an early scan, because measuring the embryo beats calculating from any remembered date, and clinical guidance says the scan wins when the two disagree.

None of which matters much once you have had an early scan, because measuring the embryo beats calculating from any remembered date, and clinical guidance says the scan wins when the two disagree.

Why this cannot settle a paternity question

A fair number of people arrive here trying to answer exactly that, and the honest answer deserves to be stated plainly rather than left for you to work out.

This calculator cannot establish who the father is, and no conception calculator can. The reasons are the ones above, stacked: the conception estimate carries several days of uncertainty on its own, ovulation moves between cycles, and sperm surviving up to five days means the fertilising encounter could have been most of a week before conception. Even a first-trimester ultrasound, which is far better than this arithmetic, still dates a pregnancy to within about five to seven days, and that window is wide enough to cover more than one possibility.

A DNA test is the only thing that answers this question, and it answers it definitively. If that is what brought you here, it is worth going straight there rather than building anything on a date from a calculator. That is not a caution about the tool being imprecise; it is that this is the wrong instrument entirely for the question.

Questions people ask

How accurate is the conception date?

Within a few days at best, and only if your cycle length and luteal phase are genuinely close to what you entered. It inherits every bit of uncertainty in those numbers, plus the ordinary cycle-to-cycle wobble in ovulation itself. It is a good estimate. It is not a measurement.

What if I conceived through IVF?

Then you do not need this, and you should not use it, because your dates are known rather than estimated. IVF pregnancies are dated from the retrieval or transfer, which is far more precise than anything derived from a period. Our IVF Due Date Calculator handles that properly.

Can I work out conception from my due date instead?

You can, roughly: subtract 266 days from the due date. Bear in mind that if the due date itself came from the 280-day rule, you are just undoing arithmetic and will land back on the day-14 assumption. If your due date was set by an early scan, working back from it is actually the better route, since the scan measured the pregnancy rather than assuming it.

References

Where the figures come from. The six-day fertile window ending on ovulation, the survival of sperm for several days, and the short viability of the egg, all of which are why conception is a window rather than a date, come from Wilcox, Weinberg and Baird's study. The two dating conventions, 280 days from the last period versus 266 from conception, and the point that the 280-day rule assumes a 28-day cycle with day-14 ovulation, come from the ACOG committee opinion on estimating the due date, which also gives the accuracy of first-trimester ultrasound dating.

1. Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine. 1995;333(23):1517-1521. https://doi.org/10.1056/NEJM199512073332301
2. American College of Obstetricians and Gynecologists. Methods for estimating the due date. Committee Opinion No. 700. Obstetrics & Gynecology. 2017;129:e150-e154. acog.org