Due date calculator
Enter the first day of your last period (or your conception date) and we'll estimate your due date, how far along you are, and your current trimester.
Your estimated due date
How this calculator works
We use Naegele's rule, the same method most care providers start with: your due date is estimated at 280 days (40 weeks) after the first day of your last menstrual period, adjusted for your average cycle length. If you calculate from a known conception date, we add 266 days (38 weeks) instead.
Remember: this is an estimate, not an appointment. Only about 5% of babies arrive on their exact due date — most come within a two-week window on either side.
Frequently asked questions
How accurate is a due date calculator?
A due date calculator gives an estimate — only about 1 in 20 babies arrives exactly on their due date. Most babies are born within two weeks before or after. Your doctor or midwife may adjust your date after an early ultrasound.
How is a due date calculated from my last period?
The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your average cycle length. If you know your conception date, 266 days are added instead.
What if my cycles are irregular?
If your cycles are irregular, the last-period method is less reliable. An early dating ultrasound (ideally before 14 weeks) is the most accurate way to estimate your due date — ask your care provider.
This tool is for general information only and is not medical advice. Your doctor or midwife will confirm your due date, usually with an early ultrasound.