The standard way to estimate a due date is Naegele's rule: 280 days (40 weeks) from the first day of your last menstrual period, assuming a regular 28-day cycle. This tool gives that estimate and roughly how many weeks along you are. A dating scan is more accurate, especially if your cycle is irregular.
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Frequently asked questions
How is the due date worked out?
It adds 280 days to the first day of your last period. This assumes a 28-day cycle with ovulation around day 14, so it can shift if your cycle differs.
How accurate is it?
It is a good estimate, but only about 5 percent of babies arrive exactly on the due date. Your midwife will usually confirm dates with an early scan.