How due dates are estimated
The standard method counts 40 weeks (280 days) from the first day of your last menstrual period. It is known as Naegele's rule, and it assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, the estimate shifts accordingly — which is why this calculator asks for your cycle length.
Why it is dated from your last period
It might seem odd to date a pregnancy from before conception, but the last period is the one reliable date most people know. Conception usually happens about two weeks later, which is why "weeks pregnant" is always roughly two weeks ahead of the time since conception.
Only about 1 in 20 babies arrive on the due date
The due date is a midpoint, not a deadline. A full-term birth is anywhere from 37 to 42 weeks, and most babies arrive in the couple of weeks around the estimate rather than exactly on it. Think of it as the centre of a likely window.
An important note
This is a general estimate for information only and is not medical advice. Your midwife or doctor can give a more accurate date, especially from an early ultrasound, and should always be your first port of call for anything about your pregnancy and health.
Frequently asked questions
How accurate is the due date?
It is a solid estimate, but an early ultrasound dating scan is usually more precise. Expect arrival within a couple of weeks either side.
What if my cycle is not 28 days?
Enter your real cycle length and the calculator adjusts the date for you.
Is this calculator free?
Yes — free, no signup, and your dates are never stored.