How it works
Enter the first day of your last period and your usual cycle length. The calculator adds 280 days (40 weeks) from that date, adjusting by the difference between your cycle and 28 days, following Naegele's rule.
Due date = LMP + 280 days + (cycle − 28)
Key dates
- Second trimester starts at 14 weeks
- Third trimester starts at 28 weeks
- Full term begins at 37 weeks (due date at 40)
Standards and sources
- Naegele's rule — 280 days from the last menstrual period
- American Pregnancy Association — due-date methods
- NHS — pregnancy due-date guidance
Last reviewed October 8, 2026
Frequently asked questions
How is the due date calculated?
Naegele's rule adds 280 days (40 weeks) to the first day of your last period, adjusted by how much your cycle differs from 28 days. For a conception date, 266 days are added instead.
How accurate is it?
Most babies are born within a few weeks of the estimate; only about 4% arrive exactly on the due date. Your clinician may refine it with an early ultrasound.
What if my cycle is irregular?
The calculation assumes a regular cycle. If yours varies, an ultrasound measurement is more reliable, so ask your clinician.