Conception Date Calculator
Enter your due date and we work backwards to estimate when you most likely conceived.
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The formula
Conception date is about 266 days before the due date (14 days after the start of the last period).
How the estimate works
Conception usually happens close to ovulation, which for a 28-day cycle is about 14 days after the first day of the last period. Because the due date sits 280 days after that period began, conception falls about 266 days before the due date.
Why it is a window, not a single day
Sperm can survive for up to five days, and an egg for about a day, so the real window is around six days. Ovulation can vary by several days from cycle to cycle too. Use the date as a best guess.
Using it with a scan
If your due date was set from an early ultrasound, the conception estimate will follow the scan, which can differ from the date you expect.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Known due date; Due date: 2026-12-01 | Estimated due date: Tuesday, December 1, 2026 |
| Known due date; Due date: 2027-02-14 | Estimated due date: Sunday, February 14, 2027 |
| Known due date; Due date: 2026-08-30 | Estimated due date: Sunday, August 30, 2026 |
Conception Date Calculator: frequently asked questions
How accurate is an estimated due date?
Only about 4 in 100 babies arrive on the due date. Most are born within two weeks either side. Treat the date as the middle of a window, and follow your midwife or doctor, who may adjust it after a scan.
Why do doctors count from the last period if conception happens two weeks later?
Because most people remember the start of their last period but not the exact day of conception. Pregnancy weeks are counted from that date, so you are already "two weeks pregnant" at conception.
Does it take leap years into account?
Yes. Calculations use real calendar dates, including leap days and the different lengths of months.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026