Due date calculator
Estimate a due date from the last period.
About this calculator
Enter the first day of your last period and your usual cycle length to see an estimated due date and how many weeks along you may be.
This is only an estimate, and few babies arrive on the exact day. Your midwife or doctor will confirm your dates, often with a scan.
How it works
Adds 280 days to the first day of the last period (Naegele's rule), adjusted for a cycle longer or shorter than 28 days.
The story behind it
Where the idea came from: Franz Naegele, German obstetrician, early 1800s.
The rule used here carries Franz Naegele's name: count 280 days, or 40 weeks, from the first day of the last period. Two centuries later it's still the usual starting point, though scans now often refine it. It gives an estimate rather than a date to expect, since few babies arrive exactly on time.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, not medical advice.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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