Reverse Due Date Calculator
Already have a due date? Enter it and we work backwards to the matching last period date and the likely conception date.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
Last period = due date - 280 days. Conception = due date - 266 days.
Counting backwards
A due date is 280 days after the first day of the last period. Subtract 280 days to get that date, and 266 days to estimate conception.
When this is useful
It helps when you were given a due date from a scan and want to see roughly where the dates fall, or if you are checking a date a clinic gave you.
Remember
The date a scan gives can differ from the date your last period suggests. Your care team usually trusts the earlier ultrasound more, and that is perfectly normal.
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 |
Reverse Due 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.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
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