period tools

Reverse Due Date Calculator

Already have a due date and wondering when conception happened? This Reverse Due Date Calculator works the pregnancy timeline backwards for you. Enter your estimated due date and we’ll show your likely conception date, a realistic conception window, and the first day of the last period your pregnancy is dated from. It’s a quick way to satisfy a curiosity, fill in a baby book, or simply understand how the 40-week countdown lines up with the days that actually mattered.

Use the due date from your doctor, midwife, or due-date calculator.

Estimated conception date

Thu, Apr 10, 1969

Most likely conception window: Sat, Apr 5, 1969 → Tue, Apr 15, 1969

Conception window

Sat, Apr 5, 1969 → Tue, Apr 15, 1969

A 5-day spread on each side, because sperm can survive several days before fertilisation.

First day of last period (LMP)

Thu, Mar 27, 1969

This is the date most pregnancies are dated from — about two weeks before conception.

Worked back from a due date of Thu, Jan 1, 1970. Estimates use the standard 40-week (280-day) pregnancy length and a 14-day gap from period to conception, so your real dates may vary by a few days.

How the Reverse Due Date Calculator works

A due date is usually set using Naegele’s rule: count 280 days (40 weeks) forward from the first day of your last menstrual period. To reverse it, we simply count back. Subtract 280 days from the due date and you land on the first day of the last period. Subtract 266 days and you land on the estimated conception date, because ovulation and conception typically happen about 14 days after a period begins.

Because the exact moment of conception is hard to pin down, we show a window of five days before and after the estimated date. Sperm can survive in the body for several days, so fertilisation can happen a little while after intercourse — that range captures the most likely days.

So for a due date of January 1st, this tool estimates conception around April 10th of the previous year, a conception window of roughly April 5th–15th, and a last period starting around March 27th.

Three dates this tool gives you

  • Estimated conception date — due date minus 266 days, the single most likely day egg met sperm.
  • Conception window — five days on each side of that date, reflecting how long sperm can survive.
  • Last menstrual period (LMP) — due date minus 280 days, the date your pregnancy is officially dated from.

These are estimates built on an average 28-day cycle with ovulation on day 14. If your own cycle runs shorter or longer, or you ovulate off schedule, the real dates can move by a few days in either direction.

When to check with your provider

A reverse due date is a fun, useful estimate — not a medical record. For anything that truly depends on exact dating, your healthcare provider is the right source. Reach out if:

  • You need a precise gestational age (an early ultrasound is the gold standard).
  • Your cycles are irregular, so the day-14 assumption may not fit you.
  • You have questions about paternity or a specific conception date that this estimate can’t answer.
  • Your due date itself has changed after a scan — re-run the tool with the updated date.

Frequently asked questions

How does a reverse due date calculator work?
It runs the standard due-date math in reverse. A due date is normally set 280 days (40 weeks) after the first day of your last period, and conception happens about 14 days after that — roughly 266 days before the due date. So we subtract 266 days from your due date to estimate conception, and 280 days to find your last period. You also get a conception window of five days on either side, since sperm can survive a few days before fertilisation.
How accurate is the estimated conception date?
Think of it as a close estimate, not a precise event. The math assumes a textbook 28-day cycle and ovulation on day 14, which is the average rather than a rule. If you ovulate earlier or later than day 14, your actual conception date shifts the same way. That is exactly why we show a window rather than a single day — your true conception date most likely falls somewhere inside it.
Why is the conception date about two weeks after my last period?
Pregnancies are dated from the first day of your last menstrual period (LMP) because that date is easy to remember, while the exact moment of conception usually is not. In an average cycle you ovulate and can conceive around two weeks after your period starts. So the LMP this tool shows is about 14 days before the estimated conception date.
Can this tell me exactly who the father is or the precise date?
No. This is a planning and curiosity tool, not a paternity or dating test. It gives an estimated range based on average cycle math. Conception can only be confirmed in a window, not pinned to a single guaranteed day. For anything that needs certainty — such as questions about paternity or precise gestational age — speak with your healthcare provider, who can use ultrasound dating.
Does this calculator store my due date?
No. Everything runs in your browser. Your due date is never sent to a server, saved to a database, or shared. Refreshing the page clears it.

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— The Period Tools Team