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How to Calculate a Pregnancy Due Date
Naegele’s rule: last menstrual period plus 280 days — educational only
Naegele’s rule estimates a due date as 280 days (40 weeks) after the first day of the last menstrual period (LMP). This tutorial walks through that calendar arithmetic and a strip from LMP to week 40. It is an educational estimate, not a clinical due date, not a diagnosis, and not medical advice. Ultrasound dating often differs.
Written by the My Calculator Stack editorial team. About our methods
Educational only — not medical advice, not a diagnosis, and not a clinical due date. Naegele’s rule (LMP + 280 days) assumes a regular 28-day cycle with ovulation near day 14. Cycle length, irregular periods, IVF dating, and ultrasound measurements can shift the estimate by days or weeks. Do not use this page to confirm pregnancy, time medications, or replace prenatal care. Talk to a qualified clinician.
1.What Naegele’s rule is
Naegele’s rule is a calendar convention: count 280 days from the first day of the last menstrual period. Gestational age in this system is dated from LMP, not from conception. Conception is often sketched near LMP + 14 days in a textbook 28-day cycle — that midpoint is educational, not a measured ovulation. The rule is not an ultrasound, not a viability assessment, and not a diagnosis of pregnancy.
2.The formula
Due date ≈ LMP + 280 days, equivalently 40 weeks from LMP. An equivalent mnemonic is LMP + 1 year − 3 months + 7 days when the calendar allows. Trimester sketches commonly group weeks 0–13, 14–27, and 28–40 from LMP. Those buckets are teaching labels, not clinical staging. Cycle length other than 28 days is not modeled here.
\[ t_{\mathrm{due}} = t_{\mathrm{LMP}} + 280\,\mathrm{d} \]
3.Worked example
Take LMP = 1 January 2026 — the same default as the pregnancy calculator. Add 280 days: 1 Jan + 280 d = 8 October 2026. The year-minus-three-months-plus-seven-days mnemonic gives the same date: 1 Jan 2027 − 3 months = 1 Oct 2026, then + 7 days = 8 Oct 2026. A conception sketch at LMP + 14 days lands on 15 January 2026. None of these dates is a clinical due date or a diagnosis.
\[ 2026\text{-}01\text{-}01 + 280\,\mathrm{d} = 2026\text{-}10\text{-}08 \]
4.Calendar-strip playground
Pick an LMP date and watch the strip mark conception (+14 days, educational), trimester boundaries, and the Naegele due date at +280 days. This is a teaching widget — no prenatal chart, no clinical chrome. Open the pregnancy calculator when you want the same 280-day arithmetic as a standalone tool, still as an estimate only.
Inline playground
Naegele · educational only
Estimated due date (Naegele)
2026-10-08
Conception sketch (LMP + 14 d, educational): 2026-01-15
2026-01-01 + 280 d = 2026-10-08
Educational only — not medical advice, not a diagnosis, not a clinical due date. Ultrasound dating and irregular cycles can differ by days or weeks. Talk to a qualified clinician for prenatal care.
5.Open the full calculator
The pregnancy calculator uses the same Naegele LMP + 280 days rule and the same 1 January 2026 example, and also reports gestational age from today. It remains an educational estimate. A clinician may date pregnancy from ultrasound or known conception instead of LMP.
Try it yourself
Open the related calculator and put these formulas to work.