Estimating a due date by hand: Naegele's rule
Most due-date estimates you'll see quoted — by an app, a calendar, a nurse doing it in her head — come from one simple rule: count 280 days forward from the first day of the last menstrual period (LMP). That's Naegele's rule, and it's arithmetic you can do with a calendar and no app at all.
The method
- Find the first day of the last menstrual period (LMP). That's day zero, not the estimated conception date.
- Add 280 days — that's 40 weeks, the standard length of a full-term pregnancy counted this way.
- Count the days forward through each month, carrying into the next month whenever you pass its last day, the same way you'd add any other run of days to a date.
- Read off the resulting calendar date. That's the estimated due date.
In one line: due date = LMP + 280 days, or equivalently LMP + 40 weeks.
A worked example
LMP: January 5, 2026.
280 days is exactly 40 weeks (280 ÷ 7 = 40, no remainder), so the fastest way to add it by hand is week by week rather than day by day. January has 31 days, so from January 5 there are 31 − 5 = 26 days left in January. That leaves 280 − 26 = 254 days to carry into February.
Stepping month by month from there: February (28 days, 2026 isn't a leap year) leaves 254 − 28 = 226; March (31) leaves 226 − 31 = 195; April (30) leaves 195 − 30 = 165; May (31) leaves 165 − 31 = 134; June (30) leaves 134 − 30 = 104; July (31) leaves 104 − 31 = 73; August (31) leaves 73 − 31 = 42; September (30) leaves 42 − 30 = 12 days into October.
12 days into October, starting the count from October 1, lands on October 12, 2026 — a Monday. Check: that's exactly 40 weeks after January 5, which is the one fact the whole calculation has to come out to.
The mistake worth watching for
The rule starts from the last period, not from conception, even though conception happens roughly two weeks later for a typical cycle. Plug in a conception date by mistake and the estimate drifts about two weeks early. The other thing worth saying plainly: Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14. Shorter or longer cycles shift the real due date without changing the arithmetic, which is exactly why clinicians often adjust the estimate with an early ultrasound rather than relying on the calendar alone.
And the estimate is just that — an estimate. Only about 1 in 20 births lands on the predicted day itself; most arrive within a week or two either side. This is a standard calculation, not a prediction anyone can promise, and not medical advice.
The free Due Date tool runs this same addition for any last-period date and returns the estimate instantly, with no arithmetic to double-check by hand.