Estimate your due date and how many weeks along you are, based on your last menstrual period.
The method behind this calculator
This calculator estimates a pregnancy due date using Naegele’s rule — about 280 days from the first day of your last period — adjusted for cycle length. It also shows the current week and trimester.
It’s an estimate only; your healthcare provider can give a more accurate date from an ultrasound.
Getting your answer
- Enter the first day of your last period.
- Adjust your average cycle length if it isn’t 28 days.
- Read the estimated due date and current week.
The formula
due date ≈ last period + 280 days (± cycle-length adjustment)
Try it with these numbers
Worked through, a cycle length of 28 produces a estimated due date of Mar 9, 2027. Underneath, Weeks along today comes out at 10.1 weeks and Trimester at First. Change any field and every figure updates as you type.
Common questions
How accurate is a due date?
It’s an estimate — only about 1 in 20 babies arrive exactly on the date. An ultrasound gives a more precise figure.
What if my cycle isn’t 28 days?
The calculator shifts the date by the difference from 28 days, which improves the estimate for longer or shorter cycles.
Should I rely on this?
Use it as a friendly guide and confirm with your doctor or midwife, who can account for your individual situation.