Conception calculator
When a baby was likely conceived, from the due date or last period.
About this calculator
When did I conceive? This conception calculator estimates the likely date of conception and the fertile days around it.
Enter your due date, or the first day of your last period and your cycle length.
How it works
Conception usually happens around ovulation, about 266 days before the due date, or about 14 days before the next period was due.
Last reviewed:
Common questions
Can I know the exact day I conceived?
Not from dates alone. Sperm can survive up to five days, so the sex that led to pregnancy may have been several days before conception. An early scan gives the closest estimate.
Is conception the same as the day I had sex?
Not always. Fertilisation happens when the egg is released, which can be days after sex, because sperm can wait in the body.
How long after conception does a pregnancy test work?
Most home tests work from around the day a period is missed, about two weeks after conception. Testing earlier can give a false negative.
Sources and further reading
The story behind it
Where the idea came from: A 1995 study of 221 women who were trying to conceive pinned down the six-day fertile window that ends on the day of ovulation.
Researchers followed couples day by day and measured hormones to find ovulation exactly. Almost all pregnancies came from sex in the five days before ovulation or on the day itself. That window is what this calculator places around your likely conception date.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, not medical advice.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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