Naegele's Rule Calculator and Its Two-Day Catch

Naegele’s rule is the classic bedside due-date formula: first day of the last period, plus 7 days, minus 3 months, plus 1 year. Enter your last period below and the calculator returns the result, along with today’s gestational age and trimester.

The rule, step by step

StatPearls states the modern form directly: adding 7 days and then 9 months to the last menstrual period — or 280 days — gives an estimated delivery date. The version that subtracts three months and adds a year is the same move done on a wall calendar, so nobody has to flip forward nine pages.

Take the search that brings many people here, a last period on March 8:

1. March 8 + 7 days gives March 15.
2. Minus 3 months gives December 15 of the previous year.
3. Plus 1 year gives December 15 of the same year the period began.

Now count it the other way. March 8 plus 280 days lands on December 13 — two days earlier.

Why the two versions disagree

Because months are not the same length. “Plus 7 days, minus 3 months, plus 1 year” is really “plus 9 calendar months and 7 days,” and nine calendar months can be anywhere from 273 to 276 days depending on which nine you cross. Add the 7 and the shortcut delivers somewhere between 280 and 283 days.

Last period	Calendar steps	280-day count	Gap
January 1	October 8	October 8	none
March 8	December 15	December 13	2 days
May 10	February 17	February 14	3 days
November 20	August 27	August 27	none

The gap is largest when the nine months span February. This is not a defect worth agonizing over — it is smaller than the uncertainty in the rule’s own assumptions — but it explains why a hand calculation and a clinic’s software can print different days for the same period date. The calculator here, like obstetric software generally, adds 280 days.

What the rule presumes about your cycle

Everything above holds only if conception happened where the rule expects it. StatPearls is explicit: the technique “assumes that the patient has a normal 28-day menstrual cycle and ovulates on day 14 of that cycle.” Neither half is guaranteed. In a pooled analysis of 3,324 U.S. cycles, ovulation was estimated to fall on day 14 in 8.7% of them, and the mean cycle was 30.3 days rather than 28.

The practical translation: if your cycles run long, real conception is later than the rule assumes and the due date it prints is early; if they run short, the reverse. Clinicians handle this not by patching the formula but by measuring the embryo — see due date by ultrasound for the discrepancies that trigger a change.

Where the rule came from, and why it survives

The formula carries the name of Franz Karl Naegele, a German obstetrician of the early 1800s. It was designed for a world with no imaging, and its virtue is that it still needs nothing but a date. StatPearls lists it under non-sonographic methods alongside uterine size and fundal height, which places it correctly: a first estimate, useful at the first visit, expected to be checked.

For the plain arithmetic without the calendar detour, use due date by last period, and to compare all four dating methods side by side, the due date calculator.

This content is informational and does not replace prenatal care. Your due date of record is set by the clinician following the pregnancy.