Best Way to Calculate Your Pregnancy Due Date

How does this due date calculator work?
Because you may not know exactly when you ovulated or conceived, a due date calculator will typically calculate your estimated due date based on your last menstrual period (LMP).

Your due date is estimated to be 40 weeks after the first day of your LMP
Your cycle is assumed to be 28 days long, with ovulation occurring at day 14
Therefore the calculator adds 280 days (40 weeks) to your LMP

This method of due date calculation is known as Naegele’s rule.

Our standard due date calculator adds 280 days (40 weeks) to the date of your last menstrual period (LMP).
My cycle isn’t 28 days. Will this due date calculator work for me?
Yes. The logic behind our pregnancy calculator works as follows:

The average cycle length is 28 days
If your cycle length is shorter, your due date will be earlier
For every day your cycle is shorter, your due date moves one day earlier
Similarly, if your cycle is longer, your due date will be later
For every day your cycle is longer, your due date moves one day later