Due date calculator
This estimate uses Naegele’s rule: 280 days, or 40 weeks, from the first day of the last menstrual period. It is a calendar estimate, not a diagnosis and not a prediction of the birth.
Many pregnancies end before or after that day. An ultrasound, cycle length, and a clinician’s exam can change the date that is used in care.
Calculator
Required fields are marked with *.
Use a comma or a dot for the decimal, as in 12.5 or 12,5. Type thousands as 1000 or 1 000. A single comma or dot is the decimal mark, so 1,000 means 1.
Results
The result will appear here.
How to use this calculator
- Enter the first day of the last menstrual period.
- Enter the date you want the week for. Use today if that is the question.
- Read the estimated due date and the completed weeks and days.
Formula
Gestational age is counted from the last menstrual period, not from conception. Week 0 is that first day.
due date = last period + 280 days
The week is the number of whole weeks from the last period to the date you chose. A note appears if that date is after the estimated due date.
Worked example
Last period 1 January 2026, week on 12 March 2026
1 January plus 280 days is 8 October 2026.
12 March is 70 days later, which is 10 weeks and 0 days.
Estimated due date 8 October 2026. On 12 March that is 10 weeks and 0 days.
Questions
Is the due date the day the baby will be born?
No. It is the day 40 weeks from the last period. Births cluster around it. Only a clinician can date a pregnancy for care.
What if the cycle is not 28 days?
Naegele’s rule assumes a 28-day cycle. A longer or shorter cycle moves the estimate. This page does not make that adjustment.
When does the count start?
On the first day of the last menstrual period, which is earlier than conception. That is why the usual length is 40 weeks rather than 38.
Sources
Last reviewed November 12, 2026