What this calculator does
It estimates a due date from the first day of your last menstrual period (LMP), using Naegele’s rule: LMP + 280 days. If your cycle is not 28 days, it adjusts the result by the difference, because a longer cycle means ovulation happens later.
It also shows how far along you are: completed weeks, days into the current week, and the trimester. Those are the numbers appointments and forms usually ask for.
The formula
Due date = LMP + 280 days + (cycle length − 28)For a 28-day cycle the adjustment is zero. For a 35-day cycle it adds seven days; for a 24-day cycle it subtracts four. The 280 days (40 weeks) is a convention: pregnancy is dated from the last period, not from conception, which adds roughly two weeks.
A worked example
Last period started 1 June 2026, cycle 28 days:
- Due date: 8 March 2027
- On 31 October 2026: 21 weeks and 5 days
- Trimester: second
With a 35-day cycle the same LMP gives 15 March 2027.
Why an ultrasound can differ
An early dating scan measures the embryo and can move the due date by several days, especially when the cycle is irregular or the LMP is uncertain. ACOG recommends using the ultrasound estimate when the two differ by more than a set threshold early in pregnancy. The calculator cannot see the scan; it only does the calendar arithmetic.
What this calculator does not do
It is not a dating scan and it does not monitor the pregnancy. It does not account for assisted conception, where the transfer date is the correct starting point. It does not tell you whether a measurement is normal. For any clinical decision, use your maternity team.
The earlier the scan, the more accurate the dating.
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. If your cycle is longer or shorter than 28 days, the calculator shifts the date by the difference, because ovulation comes later or earlier.
Is the due date exact?
No. Only about 4% of babies arrive on the estimated date. An early ultrasound is more accurate than a calendar estimate, especially with irregular cycles. Treat the result as a window, not an appointment.
What if my cycle is irregular?
A calendar estimate is weakest there. The calculator still works, but the date can move by a week or more. Use the dating scan if you have one.
Why does the page show weeks and a trimester?
Because those are the numbers used in appointments and paperwork. Weeks are counted from the first day of the last period, not from conception.