Naegele’s rule formula
Due date = first day of LMP + 7 days − 3 months + 1 year
The rule is named after Franz Karl Naegele, a German obstetrician of the early 19th century. It is equivalent to adding nine months and seven days, and assumes a 28-day cycle with ovulation on day 14.
Naegele’s rule calculation: an example
- LMP: January 10.
- Add 7 days: January 17.
- Subtract 3 months: October 17 (of the previous year).
- Add 1 year: October 17.
Counting exactly 280 days from January 10 also gives October 17 in a non-leap year. Around short months, however, the calendar shortcut and the exact count can differ by a day or two — the calculator shows both so you can see the difference.
Naegele’s rule with a longer or shorter cycle
The original rule assumes ovulation on day 14 of a 28-day cycle. A common adjustment is to add (or subtract) the difference between your cycle length and 28 days: with a 31-day cycle, add 3 days to the Naegele date; with a 25-day cycle, subtract 3 days. The calculator shows this adjustment when you change the cycle length in the main Due Date Calculator.
Limitations
Naegele’s rule does not account for cycles longer or shorter than 28 days, uncertain period dates, or ultrasound dating. Modern practice adds a cycle-length adjustment and confirms the date with an early scan. See the Estimated Due Date Calculator for the cycle adjustment and the Accurate Due Date Calculator for ultrasound dating.