The standard estimate uses Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. Only about 1 in 20 babies arrives exactly on the date — think of it as the center of a two-week window, not an appointment.
Along with the date, you get your current week of pregnancy and which trimester you are in, so you always know where you stand. The calculation happens entirely in your browser — sensitive dates are never sent anywhere.
Get your due date in 3 steps
- Open the faymox Due Date Calculator.
- Pick the first day of your last menstrual period.
- See your estimated due date, how many weeks pregnant you are now, and your current trimester.
FAQ
How accurate is the estimated due date?
It is an estimate — most babies arrive within two weeks on either side. An early ultrasound gives a more precise date; this is a solid starting point.
What if I don’t remember my last period?
Then use the date of conception if you know it (add 266 days), or ask your doctor for a dating ultrasound — the calculator needs one anchor date to work from.
What are the three trimesters?
Weeks 1–13 (first), 14–27 (second), and 28–40 (third). Each brings different milestones, which is why knowing your current week matters.