How to use the Pregnancy Due Date Calculator
- Choose whether you know the first day of your last period or the conception date.
- Enter the date (and your usual cycle length).
- Read your estimated due date, current week and trimester dates.
What does this tool do?
Doctors usually estimate the due date as 280 days (40 weeks) after the first day of the last menstrual period. This is called Naegele's rule. If your cycle is longer or shorter than 28 days, the calculator adjusts the date.
Pregnancy weeks are counted from the last period, not from conception, so at conception you're already considered about 2 weeks pregnant.
Why use it?
- Quick estimate before your first appointment.
- Shows trimester milestones.
- Adjusts for your cycle length.
Example
If your last period started on January 1, 2026, with a 28-day cycle, your estimated due date is Thursday, October 8, 2026.
The formula
Due date = First day of last period + 280 days + (cycle length − 28)
Due date = Conception date + 266 days
Accuracy and limits
- Only about 4–5% of babies are born on their due date. An early ultrasound gives the most accurate dating.
- This tool is for information only. Your midwife or doctor will confirm your dates.
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Last reviewed by the M2Toolkit team.