A Due Date Calculator

The formula this calculator uses

due date = last period + 280 days

LMP

First day of the last menstrual period

280

Days in forty weeks, the length of a typical pregnancy from the LMP

due date

LMP plus 280 days, the estimated date of delivery

How to check the result by hand

1. 1

   Identify the first day of the last period

   The count begins on the first day of bleeding, not the last day. Using the final day instead shifts the whole estimate by the length of the period, typically three to five days.

2. 2

   Add 280 days

   Forty weeks is 280 days. Counting forward from 10 February 2026 lands on 17 November 2026. Adding 40 weeks to a calendar date reaches the same place, since a week is seven days.

3. 3

   Adjust for cycle length if it is not 28 days

   Add one day for each day the usual cycle runs beyond 28, and subtract one for each day it runs short. A consistent 32 day cycle moves the estimate about four days later.

4. 4

   Work out the trimester boundaries

   The first trimester ends at the close of week 13 and the second at the close of week 27. For a last period beginning 10 February 2026 those fall on 12 May 2026 and 18 August 2026 respectively.

5. 5

   Treat the result as a window, not a deadline

   Full term spans 37 to 42 weeks, a window of roughly five weeks. The due date is the centre of that range, and a first-trimester ultrasound will often refine it further.