Calculate Due Date and Pregnancy Week

This tool applies Naegele’s rule, the most common clinical estimate: 280 days (40 weeks) counted forward from the first day of your last period. You get three things, the pregnancy week and day, the estimated due date, and the current trimester (weeks 1 to 13, 14 to 26, 27 onwards) with the days remaining. Keep in mind that this is an estimate and only a small share of babies arrive exactly on the predicted date; a first-trimester ultrasound usually gives a more accurate one. If your cycle is irregular or its length differs from 28 days, review the result with your doctor.

This tool runs entirely in your browser. No data is sent to any server.

How to use it

1. 1

   Pick the first day of your last period from the calendar.

2. 2

   Read the pregnancy week and day plus the estimated due date.

3. 3

   Check the current trimester and the days remaining in the row below.

Frequently asked questions

How accurate is the due date?

It is an estimate. Only around four percent of babies arrive exactly on the predicted day, and most births fall within two weeks either side.

What if my cycle is not 28 days?

Naegele’s rule assumes a 28-day cycle. A longer or shorter cycle shifts the estimate; a first-trimester ultrasound is the more reliable reference.

Does this replace a doctor?

No. It is a calendar calculation only. For antenatal care, ultrasound interpretation and any medical decision, see your doctor or midwife.