How to Calculate a Pregnancy Due Date by Hand (And Why Your Calculator Agrees)

If you’ve ever tested positive and immediately Googled “when is my baby due?”, you’ve probably seen a date pop up in seconds without much explanation of where it came from. Behind every modern due date calculator is a 200-year-old formula called Naegele’s rule — and you can do the math by hand in under a minute.

This article walks through that math step by step, and explains the small adjustments you should make if your cycle isn’t a textbook 28 days.

Step 1 — Find your LMP (last menstrual period)

Pregnancy is dated from the first day of bleeding of your last period before conception. That is the LMP.

Note: it’s the first day of that period, not the last. If you spotted lightly for a day before “real” bleeding started, the first day of real flow is the date that counts.

Step 2 — Add 280 days

That’s it for a textbook cycle.

Estimated due date (EDD) = LMP + 280 days (40 weeks)

For example, an LMP of January 1st gives an EDD of October 8th the same year.

If you don’t want to count days, use the older mnemonic version of Naegele’s rule:

Take the LMP.
Subtract three months.
Add seven days.
Add one year.

January 1st – 3 months = October 1st. + 7 days = October 8th. Add a year → October 8th the following year. Same answer, no calendar app required.

Step 3 — Adjust for cycle length

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles are reliably longer or shorter, ovulation happens correspondingly later or earlier — and the EDD shifts by the same amount.

The fix is simple:

Cycle adjustment = your cycle length − 28 days

Add that adjustment (positive or negative) to the 280 days. A 32-day cycle adds 4 days. A 26-day cycle subtracts 2.

Step 4 — Sanity-check with the conception date

Conception happens about 14 days after the LMP for a 28-day cycle (or 14 + cycle adjustment days). The due date should be approximately 266 days after the conception date — that’s the actual length of human pregnancy from fertilisation. If you have a known conception date from IVF or careful tracking, count 266 days forward instead.

Step 5 — Hold the result loosely

Even with perfect dates, only about 4% of babies arrive on the EDD. About 80% are born within two weeks of it. A first-trimester ultrasound is generally more accurate than the LMP method when the two disagree by more than a few days.

Try it on this site

If you’d rather skip the manual arithmetic, our Pregnancy Due Date Calculator does the same calculation, plus shows you trimester boundaries, milestones and the day of the week your due date falls on. It runs entirely in your browser — no inputs ever leave your device.

Frequently asked

What if I don’t remember my LMP?

An early ultrasound (before week 14) can date the pregnancy more precisely, since fetal size correlates closely with gestational age in the first trimester.

Does the calculator know about IVF?

For IVF, count from the day of egg retrieval (266 days) or from a day-5 transfer (261 days) to get the EDD. A conventional LMP-based calculator over-estimates by two weeks for IVF.

Why is “40 weeks” a thing if real pregnancy is 38 weeks?

Because pregnancy is dated from the LMP, two weeks before conception. Those two extra “weeks of pregnancy” are why the count starts at 40 rather than 38.

Can twins or triplets be predicted from the EDD?

No — multiple pregnancies typically deliver earlier than the calculated EDD, but the EDD itself uses the same formula. Your provider will adjust expectations once a multiple pregnancy is confirmed.

If you found this useful, the conception date calculator and ovulation calculator follow the same logic — just plug in different starting points.