Online Tool Store
Pregnancy Week Calculator
Calculate current pregnancy week, trimester, estimated due date, and days remaining from either the last period date or a known due date. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Current progress
Trimester
Estimated due date
Days remaining
How the pregnancy week calculator works
- Choose whether you're entering the first day of your last period or an already-known due date.
- The tool calculates a 280-day (40-week) pregnancy timeline from that reference point.
- It shows the current week and day, trimester, estimated due date, and days remaining.
The method
This follows Naegele's rule, the standard calendar-based estimate used for a typical 28-day cycle: 280 days after the first day of the last menstrual period.
due date = last period start date + 280 days
FAQ
How is the due date estimated?
Using Naegele's rule: 280 days (40 weeks) are added to the first day of the last menstrual period. If you enter a known due date instead, the tool works backward to find an equivalent starting date.
Why do trimesters not split evenly into three?
Trimesters are commonly defined as weeks 1-13, 14-27, and 28-40. The middle trimester is slightly longer than the first and last by convention, not an even three-way split of 280 days.
Is this a substitute for a doctor's ultrasound dating?
No. This is a calendar estimate based on typical cycle length. An ultrasound dating scan from your healthcare provider is more accurate for irregular cycles.
Is my date stored anywhere?
No. The calculation runs entirely in your browser and is not sent to a server.
How we compare
| Feature | Online Tool Store | BabyCenter | WhatToExpect |
|---|---|---|---|
| No sign-up or account | ✓ | ✗ | ✗ |
| Dates stay in this browser tab | ✓ | Not stated | Not stated |
| Free, no ads on results | ✓ | ✗ | ✗ |
For a fast, private week and due-date estimate, Pregnancy Week Calculator gets you the answer without an account.