Skip to content
MasterMath

Due Date Calculator

The estimated due date by Naegele's rule: 280 days from the first day of your last period, adjusted to your real cycle length.

Estimated due date

—

Estimated due date—
At term from—
At term until—
Estimated conception—
Right now you would be—

How this was worked out

    Indicative result. Only about 4 % of babies are born on the calculated date exactly. It is a reference, not an appointment.

    The formula

    due date = last period + 280 days + (cycle length − 28)

    Where it comes from

    Franz Naegele set out the rule in 1812 and it is still the standard: add nine months and seven days to the last period. The cycle adjustment exists because the 280 days assume you ovulated on day 14; if your cycle runs 35, you ovulated on day 21 and everything shifts a week later.

    How to work it out by hand

    1. Take the first day of your last period
    2. Add 280 days, which is forty weeks
    3. Adjust for your cycle: add the difference from 28 days
    4. The result is the estimated date

    What is worth knowing

    Only around 4 % of babies arrive on the exact calculated day. A margin of two weeks either side is normal, which is why a birth counts as “at term” between week 37 and 42. The first-trimester scan measures the embryo and often corrects this date: where they disagree, the scan wins.

    Frequently asked questions

    How is the due date calculated?

    By adding 280 days to the first day of the last period, with an adjustment if the cycle is not 28 days.

    Is the due date exact?

    No. Only 4 % of babies arrive on that exact day. The normal margin is two weeks either way.

    What is Naegele's rule?

    The classic method for estimating the birth: last period plus nine months and seven days, set out in 1812.

    What if the scan gives a different date?

    The first-trimester scan wins, since it measures the embryo directly and is more reliable than a calculation from dates.