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Likely intercourse window
Conception Calculator
Conception is the moment a sperm fertilises an egg, and it usually happens within about a day of ovulation. This conception calculator works backwards from your due date, the first day of your last period, or an ultrasound scan to estimate the date you conceived. It also shows the likely intercourse window — the days when the sex that led to the pregnancy most probably took place — along with your due date and how far along you are today. It is a calendar-based estimate, not a medical test.
Please enter a valid date.
Estimated conception date
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Last period (LMP)
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Due date
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Weeks pregnant today
How to Use the Conception Calculator
- Choose what you know: your due date, the first day of your last period, or an ultrasound scan.
- Enter the date. For a period, also enter your average cycle length; for a scan, enter the gestational age it gave you in weeks and days.
- Select Calculate to see your estimated conception date and intercourse window.
- Use the due date option if your doctor or midwife has already confirmed one — it is usually the most reliable starting point.
Conception Calculator — How It Works
Conception ≈ due date − 266 days = LMP + (cycle length − luteal phase)
Pregnancy is counted from the first day of the last period, so a full-term pregnancy is 280 days (40 weeks) long, but only about 266 days (38 weeks) of that follows conception. That is why starting from a due date, the calculator simply steps back 266 days. Starting from your last period, it adds your cycle length minus your luteal phase — 14 days for a typical 28-day cycle — which is when ovulation is expected. Starting from an ultrasound, it uses the scan’s gestational age to find the start of the pregnancy count and places conception 14 days after it.
Because sperm can survive for up to about five days and an egg only 12 to 24 hours, the likely intercourse window runs from five days before the estimated conception date up to the date itself. Any of those days could have been the one that led to the pregnancy.
Conception Calculator Example
Say your due date is 15 June. Counting back 266 days puts your estimated conception date on 22 September, and the likely intercourse window runs from 17 September to 22 September. The pregnancy count starts about two weeks before conception, on around 8 September, which is the last period date the tool shows. If you know your due date only from an early ultrasound, this is the most reliable way to work backwards to conception.
When to use this tool
Use this when you are curious about when your baby was conceived, when you want to line up a due date with your own memory of events, or when you are looking back at a pregnancy after the birth. If you are still trying to conceive, the Ovulation Calculator is the better tool, since it looks forward to your fertile window instead.
Key Features & Benefits of This Conception Calculator
- Three ways to calculate — start from a due date, a last period or an ultrasound scan, whichever you know best.
- Intercourse window — see the span of days when conception most likely happened, not just one date.
- Due date and weeks pregnant — the full picture from a single calculation.
- Instant, free and private — runs entirely in your browser, no sign-up, nothing stored.
Good to know
The result is an estimate and not medical advice. Ovulation does not always happen on the day the calendar predicts, and due dates themselves are only accurate to within about two weeks for most pregnancies, so the real conception date could be a week or more earlier or later. This tool is not suitable for determining paternity or for any legal purpose. If you have questions about your pregnancy dating, ask your doctor or midwife.
Conception Calculator — FAQs
How is the conception date calculated?
From a due date, conception is placed 266 days (38 weeks) earlier. From the first day of your last period, it is that date plus your cycle length minus your luteal phase, which is usually 14 days. From an ultrasound, the scan's gestational age is used to find the start of the pregnancy count, and conception is placed 14 days after it.
Why does the calculator show a window of several days?
Sperm can survive for up to about five days, while an egg lasts only around 12 to 24 hours after ovulation. Intercourse in the five days before ovulation, or on the day itself, could therefore have led to the pregnancy, so the tool shows that whole span rather than a single day.
How accurate is the estimated conception date?
It is an estimate. Due dates are themselves accurate to within about two weeks for most pregnancies, and ovulation can happen earlier or later than the calendar suggests, so the true date may differ by a week or more. Dating from an early ultrasound is generally more reliable than dating from a remembered period.
Why does my pregnancy count start about two weeks before conception?
Pregnancy is dated from the first day of the last period, not from conception. Conception happens around two weeks later, so a full 40-week pregnancy is about 38 weeks from conception, and you are already roughly two weeks pregnant at the moment of conception.
Can this tell me who the father is?
No. The estimate is far too imprecise to determine paternity or settle any legal question, and it should not be used for that purpose. Only a DNA test can establish paternity.
What if I conceived through IVF?
With IVF the fertilisation and embryo transfer dates are known, so they are more precise than this calendar-based estimate. Use the dates from your fertility clinic instead.