What this calculator does
Find out roughly when a pregnancy began, from a due date or a last period. Pregnancies are dated from the first day of the last menstrual period. But conception usually happens about two weeks later, around ovulation (when an egg is released). So you can estimate conception by counting back from a due date or forward from a period. You can also count forward from a known conception date to a due date.
This calculator does all three. It gives one best-guess date. When it counts back, it also shows a window of a few days on each side, because a due date or period can't tell you the exact day. Please treat the result as an estimate only. It can't show when or with whom a pregnancy began. An early ultrasound, read by a healthcare professional, is a more reliable way to date a pregnancy.
Where it's used
Why this math matters outside the classroom:
- Counting days. Working from a last period that began March 1, adding 280 days lands on December 6, the usual estimated due date.
- Cycle length. With a 32-day cycle, conception is estimated about 18 days after the last period began instead of the usual 14.
- Working backward. Counting back 266 days from a due date gives the estimated conception date that the due date implies.
- Pregnancy timeline. Expecting parents line up a likely conception window on their calendar and confirm the dates with their doctor.
- Calendar math. Students practice crossing month ends, since adding 266 days means stepping through months of 28, 30 and 31 days.
How to use it
- Choose what you'd like to find: conception from a due date, conception from your last period, or a due date from a conception date.
- Enter the date you know. For the last-period option, also enter your usual cycle length.
- If you like, change the window. That's how many days on each side of the estimate to show.
- Read the estimated date and window, or the estimated due date if you started from conception.
What the inputs mean
- Estimated due date
- Best of all, use the due date your healthcare provider gave you, especially if it changed after an ultrasound.
- Average cycle length
- Count from the first day of one period to the first day of the next. Ovulation tends to happen about 14 days before the next period. So a longer cycle moves the estimate later.
- Window either side
- A few days on each side allows for natural uncertainty. Ovulation timing varies, and sperm can survive for several days. So the day of intercourse and the day of conception may differ.
Formula and method
From a due date: conception ≈ due date − 266 days From the last period: conception ≈ LMP + (cycle length − 14) days Reverse: due date ≈ conception + 266 days Window = estimate ± chosen number of days
LMP stands for the first day of your last menstrual period. Cycle length is the number of days from one period to the next. The ≈ sign means "about", and ± means "plus or minus".
A standard pregnancy lasts 280 days from the first day of the last period. With a 28-day cycle, ovulation and conception happen about 14 days in. That leaves 266 days from conception to the due date. Both the counting-back and counting-forward options use this.
For the last-period option, the calculator assumes ovulation comes about 14 days before your next expected period. So the estimate falls cycle length minus 14 days after your period started. That's day 14 for a 28-day cycle and day 21 for a 35-day cycle. All the math uses whole calendar days, so time zones and daylight saving don't affect it.
Worked example
The estimated due date is Thursday, October 8, 2026. When did conception most likely happen?
- Days from conception to due date: 280 − 14 = 266.
- Oct 8, 2026 − 266 days = Thursday, Jan 15, 2026.
- With a window of 3 days either side: Jan 12, 2026 to Jan 18, 2026.
Result: Conception most likely happened around Thursday, Jan 15, 2026, within a window of Jan 12 to Jan 18, 2026.
Understanding the result
The estimated date is the middle of a range, not an exact answer. Even with regular cycles, ovulation can shift by several days from month to month. Due dates are estimates too.
If an early ultrasound set your due date, a conception estimate based on it is usually more reliable than one from your last period alone.
When you start from a conception date, the due date is only as accurate as the date you enter. If you know the date of an IVF transfer instead, the due date calculator has its own option for that.
Assumptions and limitations
- A standard pregnancy of 280 days, counted from the last period.
- Ovulation and conception happen about 14 days before the next expected period.
- Dates are whole calendar days.
- It can't tell the exact day of conception. Please don't use it to establish paternity or for any legal purpose.
- Irregular cycles, recent hormonal contraception or breastfeeding can make the last-period option unreliable.
- It doesn't allow for pregnancies that an ultrasound dates differently. Follow the dates your healthcare provider gives you.
Frequently asked questions
Can this tell me the exact day I conceived?
No. It gives the most likely date and a window around it. Ovulation timing varies, sperm can survive for several days, and due dates are estimates. So the true date could be a few days either side.
Why is it 266 days and not 280?
The 280 days of a standard pregnancy are counted from the last period, which is about two weeks before conception. Take away those 14 days and you're left with 266 days from conception to the due date.
What window should I use?
Three days on each side is a good starting point for a regular cycle. Use a wider window if your cycles are irregular or you're unsure of your dates.
How is this different from the due date calculator?
The due date calculator counts forward from a period, conception or IVF transfer. It also shows how far along a pregnancy is. This calculator focuses on the conception date and the window around it. It also gives a quick due date from conception.
How this calculator is checked
Method: Standard obstetric dating: due date = LMP + 280 days; conception ≈ LMP + (cycle − 14) days; conception to due date = 266 days. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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