How to use the Conception Calculator
- Choose Plan ahead and enter the first day of your last period.
- Select your average cycle length.
- Press the Calculate button.
- Read the days when conception is possible and most likely, and the due date if you conceive then.
- Use Check a date if you want to see whether a past date of intercourse fell in your fertile window.
Conception Date Calculator: Find Your Due Date From the Day You Conceived
What Is a Conception Date Calculator?
A conception date calculator estimates your due date starting from the day your baby was conceived. It suits you if you tracked ovulation, know roughly when you got pregnant, or had a treatment such as IUI. You enter one date, and the tool builds your whole pregnancy timeline from it.
How the Conception Date Calculator Works
The calculator adds 266 days (38 weeks) to your conception date, which is the average length of pregnancy measured from conception to birth.
Doctors, however, count pregnancy from the first day of your last period, and conception normally happens about two weeks after that. To match the way your doctor counts, the calculator adds those two weeks when it shows how far along you are.
Example: if you conceived on 12 October 2026, your due date is 5 July 2027. On the day of conception you are already counted as 2 weeks pregnant.
Besides your due date, the tool can also show you:
- how many weeks and days pregnant you are today
- which trimester you are in
- how many days are left until your due date
- how far you are through the 40 weeks
How to Find Your Conception Date
Few people know the exact day they conceived, but you can still make a good estimate. These methods are listed from most to least reliable:
| Method | How to use it |
|---|---|
| Ovulation (LH) test | Ovulation usually follows within a day or two of a positive test, so use that day or the next one. |
| Temperature or cycle tracking | Your basal body temperature rises slightly after you ovulate, which shows when it happened. |
| Counting back from your period | Ovulation typically falls about 14 days before your next period would start. |
| Date of intercourse | Use with care. Sperm can live up to five days, while an egg survives only 12 to 24 hours, so conception may happen days after sex. |
If none of these fit your situation, use our due date calculator with the first day of your last period instead. You can also ask your doctor for an early scan to confirm your dates.
Conception Date vs. Last Period Date
With regular cycles, both methods give almost the same due date. The difference is where each one starts. The last period method assumes a 28-day cycle with ovulation on day 14. The conception method starts from the actual event.
So if your cycles are long, short or irregular, the conception date is often the better choice, provided you know when you ovulated. Our guide on how a due date calculator works explains this in more detail.
How Accurate Is Your Result?
Your result is an estimate. Only about 1 baby in 20 arrives on the due date, and a birth anywhere between 37 and 42 weeks is considered normal. If your conception date is a guess, it can be off by several days, and your due date will be off by the same amount.
That is why you should always confirm your date with your doctor. A dating ultrasound in the first trimester is the most reliable check. After your scan, compare the result with our ultrasound due date calculator.
If you conceived through IVF, use the IVF due date calculator instead, because the age of the embryo on transfer day changes the count.
What to Do After You Know Your Due Date
- Book your first prenatal visit. Your provider will confirm your dates.
- Ask about an early dating scan. It gives the most dependable due date.
- Check your starting weight with the pre-pregnancy BMI calculator.
- Follow your progress. Our pregnancy calculator shows how far along you are, week by week.
Come back to the conception date calculator above whenever you want to recheck your dates.
Disclaimer: This article is for general information only and is not medical advice. Results are estimates, not guarantees. Always follow the guidance of your doctor or midwife.
Conception Calculator FAQs
What is the fertile window?
It is the 5 days before ovulation plus the day of ovulation. Sperm can live for up to 5 days, and an egg survives for about 12 to 24 hours.
Which days are the most fertile?
The last 3 days of the window, meaning the two days before ovulation and ovulation day, are the most fertile. Sex on these days leads to pregnancy in roughly 3 in 10 cases.
How is the conception date estimated?
Ovulation is estimated at your cycle length minus 14 days after the first day of your last period. The most likely conception dates are the 3 days up to and including that day.
How is the due date worked out?
The due date is 266 days after conception, which is 280 days from the start of your last period in a 28-day cycle.
How often should we have sex when trying to conceive?
Having sex every 1 to 2 days, and especially in the fertile window, gives the best chance. You do not need to time it exactly.
Does this work with irregular cycles?
Only roughly. If your cycles vary a lot, ovulation tests, basal body temperature or cervical mucus checks give a better signal.