Conception Calculator guide
Conception Calculator: When Did I Conceive?
Use our conception calculator to estimate when you conceived from your due date, the first day of your last period or your baby's date of birth.
By the AllCalculator editorial teamUpdated 16 October 20262 min read
Pregnancy is dated from the first day of your last period, not from conception, so working out when you actually conceived takes a bit of counting. Our Conception Calculator does it for you.
How to use the calculator (Conception Calculator)
Choose what you know:
- Due date: from your dating scan, if you've had one. This is the most accurate.
- Last period: the first day of your last period, and your usual cycle length.
- Birth date: your baby's date of birth and how many weeks pregnant you were.
You'll see your estimated conception date, the likely range around it, your fertile window and your due date.
How it works
- Conception usually happens within a day of ovulation, which is about 14 days before your next period. In a 28-day cycle, that's about 2 weeks after the first day of your last period.
- A baby is due about 266 days (38 weeks) after conception, or 280 days (40 weeks) after the first day of the last period.
So the calculator counts back 266 days from your due date, or forward from your last period, adjusted for your cycle length.
A worked example
If your due date is 1 March 2027, your estimated conception date is Monday 8 June 2026, most likely between 5 and 11 June.
Why it's only an estimate
Sperm can survive in the body for up to 7 days, and ovulation doesn't always happen on the same day each cycle. So sex up to a week before ovulation can lead to pregnancy. A dating scan in the first trimester is the most accurate way to date a pregnancy, but even that can't pinpoint the day you conceived.
Frequently asked questions
Can this tell me who the father is?
No. Because sperm can survive for several days and cycles vary, a conception date can't prove when or with whom a baby was conceived. Only a DNA test can do that.
Why does my scan date differ from my period date?
If your cycle is longer or shorter than 28 days, or you ovulated earlier or later than usual, your scan date may be different. Your midwife will usually use the scan date.
My baby was born early. Does that change the conception date?
Enter how many weeks pregnant you were at birth, and the calculator will count back from that.
For more about your pregnancy, see our Pregnancy Calculator and How Many Weeks Pregnant Calculator.
Sources and further reading
- NHS: Due date calculatornhs.uk
- NHS: Fertility in the menstrual cyclenhs.uk
- NHS: Trying for a babynhs.uk
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.