Pregnancy Due Date Calculator
The estimated due date from the first day of the last menstrual period, by Naegele's rule.
Results
What this tool does
Franz Naegele's rule has been used since the early nineteenth century: count forty weeks, or 280 days, from the first day of the last period. It assumes a textbook cycle, which is why a dating scan usually adjusts it. Change the number of days if a scan or a known cycle length gives you a different figure to count from.
Formula
estimated due date = first day of the last period + 280 days (Naegele's rule)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
start_date | First day of the last period | — |
add_days | Days of gestation | — |
date | Estimated due date | — |
weekday | Day of the week | — |
week | ISO week number | — |
Worked example
- First day of the last period2026-01-15
- Days of gestation280
- Estimated due date22 Oct 2026
- Day of the week4
- ISO week number43
Limitations
- This tool is informational only. It does not diagnose, treat or replace a qualified health professional. Speak to a doctor before making decisions about your health.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Dates are handled as calendar dates. Time zones, daylight saving changes and public holidays are not taken into account.
Frequently asked questions
How accurate is the 280-day rule?
It is a convention, not a prediction. Naegele's rule assumes a regular 28-day cycle with ovulation on day 14, and only a small minority of babies arrive on the calculated date — most are born within two weeks either side of it. An early ultrasound dates a pregnancy more reliably than the last period does, and any real decision belongs with a midwife or doctor, never with a calculator.