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- Estimates your due date from the first day of your last period.
- Or from the date of conception.
- Adjusts for cycles longer or shorter than 28 days.
- Shows how far along you are and a trimester timeline.
How to use the due date calculator
Choose your method
Last period or conception date.
Enter the date
The first day of your last period, or the conception date.
Enter your cycle length
Usually 28 days; change it if your cycle is regularly longer or shorter.
Read your estimated due date
With your current week and key milestones.
How the due date is estimated
Most due dates are worked out with Naegele’s rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation around day 14. For longer or shorter cycles, the date moves by the difference. From a known conception date, the due date is 266 days later.
Due date formulas
Due date = first day of last period + 280 days
Adjusted for cycle length: + (cycle length − 28) days
From conception: conception date + 266 days
Example: last period started September 1, 2026
28-day cycle.
- Estimated conception September 15, 2026
- Second trimester starts December 8, 2026
- Third trimester starts March 16, 2027
- Full term from May 18, 2027
- Estimated due date Tuesday, June 8, 2027
With a 32-day cycle, the estimate moves to June 12, 2027.
Pregnancy milestones
First trimester
Weeks 1 to 13.
Second trimester
Weeks 14 to 27.
Third trimester
Week 28 to birth.
Full term
From 37 weeks.
This is an estimate for information only. Only a small share of babies arrive exactly on their due date, and an early ultrasound may give a different date. Always follow your doctor or midwife’s advice.
Due date calculator FAQ
How is a due date calculated?
By adding 280 days to the first day of your last period, adjusted for your cycle length.
How accurate is a due date?
It is an estimate. Most babies are born within about two weeks either side of it.
Can my due date change?
Yes. An early ultrasound measurement is often used to confirm or adjust it.
What if my cycle is not 28 days?
Enter your usual cycle length and the due date moves by the difference from 28 days.
How many weeks is a full-term pregnancy?
Full term is from 37 weeks; the due date is at 40 weeks.
Can I calculate from the conception date?
Yes. The due date is about 266 days after conception.