Naegele's rule
Due date = first day of last period + 280 days (40 weeks), ± (cycle length − 28)
The estimate counts 280 days from your LMP and nudges for cycle length, since ovulation timing shifts. Only about 4% of babies arrive exactly on the due date — most come within a week either side. A dating ultrasound is more precise.
How your due date is estimated
The most common method, Naegele's rule, counts 280 days (40 weeks) forward from the first day of your last menstrual period. Because ovulation timing shifts with cycle length, the calculator nudges the date for cycles longer or shorter than 28 days, and also shows how many weeks along you are and your current trimester.
This is an estimate for general information only and is not medical advice. Please confirm with your healthcare provider.