Due Date Calculator
About the Due Date Calculator
Your due date is an estimate of when your baby is likely to be born, calculated from the start of your last menstrual period.
Formula Used
How It Works
This calculator adds 280 days to your last period date, then adjusts slightly if your average cycle length is longer or shorter than the standard 28 days.
Step by Step
- Enter the first day of your last period.
- Select your average cycle length.
- The calculator adjusts the standard 280-day estimate based on how your cycle differs from 28 days.
Worked Example
Example: LMP 1 January with a 30-day cycle (+2 days adjustment):
Due Date ≈ 1 January + 282 days = 10 October
Frequently Asked Questions
What is Naegele's rule?
It's the standard method doctors use to estimate a due date: adding one year, subtracting three months, and adding seven days to the first day of your last period - equivalent to LMP + 280 days.
Why does cycle length matter?
Ovulation timing shifts with cycle length, so a longer or shorter cycle than 28 days moves the estimated conception and due date accordingly.
Will my baby be born exactly on this date?
Unlikely - due dates are estimates, and full-term babies can arrive anywhere from about 3 weeks before to 2 weeks after this date.