Due date calculator
Work out your estimated due date from the first day of your last period, or from a known conception date. Cycle length is taken into account.
The maths, openly
We do not hide the calculation. Here is exactly what runs:
- From your last period: due date = first day of last period + 280 days, then adjusted by (your cycle length − 28). Conception is estimated at day 14 of your cycle, adjusted the same way.
- From a conception date: due date = conception date + 266 days.
- How far along: counted from the last-period date, in completed weeks. First trimester runs to week 13, second to week 27, third from week 28.
The 280-day figure is Naegele's rule, which has been the standard since the nineteenth century and remains what most clinicians use as a starting point.
Frequently asked questions
How accurate is a due date calculator?
It gives a good estimate, but only about one birth in twenty happens on the estimated date. Most babies arrive within two weeks either side. A first-trimester ultrasound is more accurate because it measures the baby directly rather than inferring from your cycle.
Why does pregnancy count from my last period?
Because the first day of your last period is a date you are likely to know, while the date of conception usually is not. It is the convention clinicians use, and it means you are counted as about two weeks pregnant at the moment of conception.
What if my cycle is not 28 days?
Enter your actual average cycle length. The standard 280-day rule assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts both the conception date and the due date. Our calculator adjusts for this.
My scan gave a different date. Which should I use?
Use the scan. A dating ultrasound in the first trimester is the most accurate method available, and your care team will date the pregnancy from it.
Next: find out which week you are in, or read how due dates are calculated in more depth.