Due date calculator
A due date from the first day of the last period. Naegele's rule is two different rules that disagree by up to three days, and about four per cent of babies arrive on the day. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
This is arithmetic, not medical advice. A dating scan beats this arithmetic and is what your midwife will use. This assumes a regular cycle with ovulation on day 14, which is true of a minority of people, and it says nothing whatever about the pregnancy. The rule dates from 1812; the accuracy figures were checked against the NHS on 21 September 2026.
Naegele's rule is two different rules
Both are called Naegele's rule and they disagree.
| As it is taught | Add a year, subtract three months, add seven days. |
|---|---|
| As it is coded | Add 280 days. |
Those are not the same thing, because months are not the same length. Swept across every start date in four years, they agree on only 44% of them and differ by one, two or three days on the rest. Almost nothing mentions this, and most calculators quote the first rule in their explanation and implement the second.
This page gives both, and the gap.
About four per cent of babies arrive on the day
That is the number worth carrying away. 80% of births happen between 37 and 41 weeks, and the standard deviation around the estimate is about 16 days when it comes from the last period and 14 days from a first-trimester scan — which is why the NHS replaces this estimate with the scan's as soon as it has one.
A due date is the middle of a range, and the range is about five weeks wide. This page shows it as a window rather than a date.
The 280 days assumes a 28-day cycle
Naegele's rule, from 1812, assumes a 28-day cycle with ovulation on day 14. Cycles between 21 and 35 days are all ordinary, and a longer cycle means later ovulation — so the estimate moves later, day for day. A 35-day cycle puts the due date a week beyond what the plain rule says.
Most calculators do not ask. This one does, and shows both the adjusted and the unadjusted date so you can see what the adjustment did.
Why pregnancy is counted from before it started
Weeks are counted from the first day of the last period, not from conception — so at the moment of conception you are already notionally about two weeks pregnant. It is an odd convention and it exists for a simple reason: the date of the last period is a thing people know, and the date of conception generally is not.
So “40 weeks” is about 38 weeks of actual gestation, and the two-week difference is built into every figure on this page.
What it will not do
- It will not beat a dating scan. A first-trimester ultrasound is a better estimate than any arithmetic from a remembered date, and it is what your midwife will use.
- It will not account for an irregular cycle, IVF, or anything else that breaks the assumption of ovulation on day 14 of a predictable cycle.
- It will not tell you anything about the pregnancy. It is a date calculation.