Due date calculator
Find out when your baby is likely to arrive and check the most important dates of your pregnancy.
Due date calculator: what it is and how it works
The estimated due date is traditionally calculated with Naegele’s rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. The rule assumes a 28-day cycle with ovulation on day 14; if your cycles are longer or shorter, the calculator adjusts the date accordingly. If you know the conception date, 266 days are added.
Only around 4–5 % of babies are born exactly on their due date: birth between weeks 37 and 42 is considered normal. The first-trimester scan dates the pregnancy more accurately, and your midwife or doctor may adjust the date based on it.
How to use it
- Choose whether you know the date of your last period or of conception.
- Enter the date and, if relevant, your cycle length.
- See your estimated due date and the key dates.
Due date = last period + 280 days + (cycle length − 28) · or conception + 266 daysWorked example
If your last period started on 1 August 2026 and your cycle is 28 days, your estimated due date is Saturday 8 May 2027.
Frequently asked questions
What is Naegele’s rule?
The classic method to estimate the due date: add one year and seven days to the first day of the last period and subtract three months, which equals adding 280 days.
Is the due date exact?
No. It is an estimate: most births happen between weeks 38 and 41.
What if my cycles are irregular?
The estimate is less reliable. The first-trimester scan is the best reference.
How are pregnancy weeks counted?
From the first day of the last period, even though conception happens about two weeks later.