How is a due date calculated?

Add 280 days (40 weeks) to the first day of your last period, or subtract 3 months and add 7 days.

How accurate is a due date?

It's an estimate; only about 1 in 20 babies are born on it, and full term is anywhere from 37 to 42 weeks.

What if my cycle isn't 28 days?

Shift the date by the difference; a dating ultrasound gives a more reliable estimate.

How to Calculate Your Due Date

Published 10/29/2025 · 3 min read · Health calculators

Sofia Nunes — Health & wellness writer at Allin

Nutrition · Hydration

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In short

The standard estimate adds 280 days (40 weeks) to the first day of your last menstrual period — this is Naegele's rule. A quick shortcut is to take that date, subtract three months and add seven days. For a period starting 1 January, that gives an estimated due date around 8 October. It's an estimate: only about 1 in 20 babies arrive on the exact day.

A woman holding up a small baby garment. — Matilda Wormwood · Pexels · Pexels

Naegele's rule adds 280 days to your last period — or subtract three months and add seven days. Here's how it works and why it's only an estimate.

Naegele's rule

The standard way to estimate a due date is Naegele's rule: add 280 days — exactly 40 weeks — to the first day of your last menstrual period. Pregnancy is measured from that day rather than from conception because it is the last date most people can pin down. Those 40 weeks are why pregnancy is described as nine-ish months: it is counted in weeks from the last period, not from when the baby was conceived.

The quick shortcut

Adding 280 days in your head is awkward, so there is a neat shortcut that gives the same answer: take the first day of your last period, subtract three months, then add seven days. If your last period began on 1 January, go back to 1 October and add a week for 8 October. It works because three months back plus a week lands almost exactly 40 weeks forward on the calendar.

Why cycle length matters

Naegele's rule quietly assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, you likely ovulated later or earlier, and the true due date shifts by that difference. Someone with a regular 35-day cycle, for instance, might be due about a week later than the rule suggests. This is one reason an early ultrasound, which measures the baby directly, is trusted over the calendar when the two disagree.

It's only an estimate

The due date is a target, not a deadline. Only around 4% of babies are born on the exact date, and a full-term birth is anything from 37 to 42 weeks — a five-week window. Most arrive within a fortnight either side. Treat the date as the middle of a range to plan around, keep your antenatal appointments, and let your care team update the estimate as the pregnancy progresses.

Due date calculatorEstimate your baby's due date (Naegele's rule).Try the tool →

Frequently asked questions

How is a due date calculated?: Add 280 days (40 weeks) to the first day of your last period, or subtract 3 months and add 7 days.
How accurate is a due date?: It's an estimate; only about 1 in 20 babies are born on it, and full term is anywhere from 37 to 42 weeks.
What if my cycle isn't 28 days?: Shift the date by the difference; a dating ultrasound gives a more reliable estimate.

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This is general information, not medical advice. Every pregnancy is different; follow the guidance of your doctor or midwife.

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