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Due date calculator

Get the due date Naegele’s rule gives from the first day of the last period. It’s worked out on your device.

days

The date to count the weeks to.

Result

Your result will appear here.

What it does

The due date calculator applies Naegele’s rule to the first day of the last period and shows:

The answer updates as soon as you change a date or the cycle length.

How to use it

  1. Enter the first day of the last period. It starts 70 days before today, as an example.
  2. Change the cycle length if it isn’t 28 days. It goes from 20 to 45 days.
  3. Weeks on starts as today. Change it to count the weeks to another date.
  4. Select Copy result to put the answer on your clipboard.

Nothing you enter is kept: after a reload the dates and the cycle length start again from their example values.

How it’s calculated

Due date = first day of last period + 280 days + (cycle length − 28 days). With a period starting on 30 December 2025 and a 28-day cycle: 30 December 2025 + 280 days + (28 − 28) = 6 October 2026.

Weeks = chosen date − first day of last period − (cycle length − 28 days), shown in weeks and days. On 10 March 2026 that is 70 days, or 10 weeks.

Good to know

Privacy

Everything is worked out in your browser, on your device. Nothing you type is uploaded, and neither the dates nor the cycle length are stored.

Frequently asked questions

What is Naegele’s rule?

Naegele’s rule estimates a due date as 280 days, or 40 weeks, after the first day of the last menstrual period. It dates from the 19th century and is still the usual way to give a first estimate. Pregnancy weeks are counted from that same day, so 2 of the 40 weeks come before conception.

Why does the cycle length matter?

The rule takes a 28-day cycle, with ovulation about 14 days after the period starts. With a longer cycle ovulation tends to come later, and with a shorter one earlier, so the calculator moves the due date, the conception date and the week count by the cycle length minus 28 days. A 30-day cycle moves them 2 days later.

Will the baby arrive on the due date?

Usually not on the day itself. Few births fall on the estimated date: it marks the middle of the weeks in which a birth at term is expected. It is an estimate, not a fixed date.

What does Weeks on do?

It sets the date the weeks, trimester and days to go are counted to. It starts as today; change it to see the week on another date. Before the period date, or when the counted weeks (adjusted for the cycle) fall below 0 or past 44, those rows read Outside 0 to 44 weeks.