Conception Calculator: When Did I Conceive?
Estimate the date you conceived from your due date or the first day of your last period. Conception is about 266 days before the due date. Runs in your browser.
Enter a date to see your estimated due date.
About the Conception Calculator
The Conception Calculator estimates the date a pregnancy began. It opens on the Due date method: enter the estimated due date from your midwife, doctor or scan report, and the calculator works back to the likely conception date and the equivalent first day of the last period. You can also start from the first day of your last period, a known conception date or an IVF transfer date. Alongside the conception date you see how many weeks pregnant you are today, the trimester and the key milestones.
Dates are counted as whole calendar days, so the result is the same in every time zone.
This is an estimate, not medical advice. Conception cannot be pinned to a single day from dates alone: ovulation varies, sperm can survive for several days, and a due date is itself an estimate. A dating scan in the first trimester is the most accurate guide. The calculator cannot establish paternity.
Why conception is about 266 days before the due date
Counting from the last period
Pregnancy is dated from the first day of the last menstrual period, not from conception, because that day is usually known while conception is not. A due date is set 280 days, or 40 weeks, after that day, following Naegele's rule.
Ovulation two weeks later
In a typical 28-day cycle ovulation, and so conception, happens about 14 days after the period starts. That puts conception 280 − 14 = 266 days, or 38 weeks, before the due date. In longer or shorter cycles ovulation moves, which is why the last-period method asks for your cycle length.
The formula
From a last period the calculator first adds 280 days plus any difference between your cycle and 28 days to find the due date, then works back in the same way.
Worked example
With a due date of Thursday, 8 October 2026, the estimated conception date is Thursday, 15 January 2026, and the equivalent last period is 1 January 2026. Starting from a last period on 1 January 2026 with a 28-day cycle gives the same answer.
How to use it
- Keep Due date selected, or choose another starting point.
- Enter the date.
- Read the estimated conception date and the progress below it.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. The dates are never sent to a server, nothing is stored, and closing the page clears them.
Related tools
The full Due Date Calculator shows the milestone list for the whole pregnancy, the Pregnancy Calculator answers how many weeks pregnant you are, and the Date Calculator counts the days between any two dates.
Sources
- Naegele's rule: due date = first day of the last period + 280 days, as used in obstetric dating; see the American College of Obstetricians and Gynecologists, Methods for Estimating the Due Date, Committee Opinion 700, 2017.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-1521.
Open-source credits
The calculations are implemented in-house in TypeScript, with no third-party libraries. They follow Naegele's rule (280 days from the last period) and its conception form (266 days from conception to the due date).
Frequently asked questions
How is the conception date worked out from the due date?
The calculator subtracts 266 days, or 38 weeks, from the due date. A due date is set 280 days after the first day of the last period, and conception usually happens about 14 days after that day, so 280 minus 14 gives 266.
Why is conception about two weeks after the last period?
Conception happens around ovulation, and in a typical 28-day cycle ovulation is about 14 days after the period starts. Pregnancy is still counted from the last period, so at conception a pregnancy is already described as two weeks along.
How accurate is the estimated conception date?
It is an estimate, not medical advice. Ovulation can come days earlier or later than average, sperm can survive for up to five days, and a due date is itself an estimate, so the real date may differ by a week or more. An early dating scan is the most reliable guide.
Can the conception date tell who the father is?
No. Because conception cannot be pinned to a single day from dates alone, a calendar estimate cannot establish paternity. Only a DNA test can do that.
Can I start from my last period instead?
Yes. Choose Last period, enter the first day of your last period and your usual cycle length, and the calculator finds the due date and works back to the conception date. Conception and IVF transfer dates are also accepted.