Conception Calculator: When Did I Conceive?
Work back from your due date, or forward from your last period, to estimate the day you conceived and the days the sex that led to it most likely happened.
Conception usually happens about two weeks after the first day of your last period, so it falls about 266 days before your due date. A due date of June 15, 2027 points to conception around September 22, 2026. Sperm can survive several days, so the sex that led to it most likely happened in the six days ending on that date, September 17 to 22.
This calculator gives a calendar estimate for general education only. It is not medical advice, it cannot confirm when a pregnancy began, and it cannot establish paternity. Ovulation can happen earlier or later than any estimate, and an early ultrasound dates a pregnancy more closely. For questions about dating a pregnancy or your care, talk to a doctor, midwife or other qualified health professional.
Estimated conception date
Tue, Sep 22, 2026
- Sex most likely happened between
- Thu, Sep 17, 2026 and Tue, Sep 22, 2026
- Estimated due date
- Tue, Jun 15, 2027
- Matching last period
- Tue, Sep 8, 2026
Embed this calculator on your site →
About the Conception Calculator
Due dates are counted from the first day of the last period, not from conception, and a full-term pregnancy is dated as 280 days, or 40 weeks, from that day. In a 28-day cycle an egg is released about 14 days after the period starts, and conception can only happen in the day or so after that. So conception sits two weeks into the count, and 280 - 14 = 266 days, or 38 weeks, before the due date. Working back from a due date subtracts those 266 days. Working forward from a period adds your cycle length and takes away the 14 days between release of the egg and the next period, which is the steadier half of the cycle. A 35-day cycle therefore moves conception, and the due date with it, a week later than a 28-day cycle would. The six-day window reflects how long sperm can wait: sex on any of the five days before the egg is released, or on that day, can lead to a pregnancy, and sex after it almost never does. If your due date came from an early ultrasound rather than from your period, the back-count is usually closer, because the scan measures the pregnancy itself.
Frequently asked questions
How do I work out when I conceived from my due date?+
Count back 266 days, which is 38 weeks. A due date of June 15, 2027 gives a conception date around September 22, 2026. The sex that led to it most likely happened between September 17 and September 22, because sperm can live in the body for up to five days before the egg is released.
Can a conception calculator tell who the father is?+
No. The window is six days wide at best, and in a given month ovulation can land several days away from the calendar estimate, so two encounters a week or more apart can both fall inside a plausible range. Dates can make one partner more or less likely, but only a DNA test, which can be done during pregnancy or after birth, settles paternity.
Why is my due date 40 weeks when pregnancy lasts 38?+
Pregnancy is dated from the first day of your last period because that day is easy to know, while the day an egg was fertilized usually is not. The first two of those 40 weeks come before conception, so the time from conception to the due date is about 38 weeks, or 266 days.
What if my cycles are longer or shorter than 28 days?+
Use the last-period option and enter your usual cycle length. The calculator assumes the egg is released 14 days before your next period would have started, so a 32-day cycle puts conception on day 18 and moves the due date four days later than the standard count. If your cycles vary a lot from month to month, a date from an early ultrasound will be closer than either calendar method.
How accurate is the estimated conception date?+
Treat it as the middle of a range of several days rather than a single day. Ovulation shifts with stress, illness and travel, and due dates themselves are estimates: most babies arrive within a week or two either side of theirs.
Embed this calculator
Free to use on your own site. Paste this snippet where you want the conception calculator to appear — it resizes itself automatically.
<iframe
src="https://manytools.co/embed/conception-calculator"
title="Conception Calculator by ManyTools"
data-manytools="conception-calculator"
width="100%"
height="640"
loading="lazy"
style="border:0;width:100%;max-width:720px"></iframe>
<script>
(function () {
window.addEventListener("message", function (e) {
if (e.origin !== "https://manytools.co") return;
var d = e.data || {};
if (d.type !== "manytools:embed:height" || d.slug !== "conception-calculator") return;
var f = document.querySelector('iframe[data-manytools="conception-calculator"]');
if (f && d.height) f.style.height = d.height + "px";
});
})();
</script><a href="https://manytools.co/health/conception-calculator">Conception Calculator by ManyTools</a>