Due Date Calculator
Naegele's rule from the LMP — the estimated date, the cycle adjustment stated, and where the count stands today.
Due Date Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- LMP method — no ultrasound inputs
- Cycle adjustment is a stated convention, 21–35 days
In short: An LMP of Tuesday, 20 May 2026 reads a due date of Wednesday, 24 February 2027 — 280 days from the last menstrual period at a 28-day cycle, Naegele's rule exactly as obstetrics runs it. On the as-of date the count stands at 17 weeks 2 days gestational with 159 days to go. The cycle adjustment is stated, not smuggled: a 24-day cycle moves the date 4 days earlier, a 33-day one 5 days later — and ultrasound dating remains the more accurate ruler when the two disagree.
Formula
EDD = LMP + 280 days + (cycle − 28)
Naegele's rule adds 280 days — 40 weeks — to the first day of the last menstrual period, assuming a 28-day cycle with ovulation near day 14. The cycle term moves the date by the difference: shorter cycles pull it earlier, longer ones push it later, one day per day. Gestational age counts from the LMP rather than conception — the two-week head start is the convention every week count uses — and the estimate stays an estimate: about 4% arrive on the day, the honest window is 38 to 42 weeks.
Worked Example
- Enter the LMP, the cycle length and the as-of date.
- Read the due date and the cycle adjustment.
- Check the standing count against the as-of.
Defaults: LMP 20 May 2026 → due Wednesday, 24 February 2027; 17 weeks 2 days on the as-of, 159 days to go.
Strengths & Limits Of This Model
Where this engine is strong
- The cycle adjustment stated, not smuggled
- The 38–42 window printed beside the date
Where it stops
- One LMP, one cycle per run
Practical Use Cases
First appointment
the date the midwife will write
Planning
the 38-to-42 window around the date
Cycle honesty
the adjustment stated, not guessed
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Due Date Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is Naegele's rule?
The standard dating convention: add 280 days to the first day of the last menstrual period — the same thing as one year, minus three months, plus seven days. It assumes a 28-day cycle with ovulation near day 14, which is why the cycle adjustment sits right beside it on this page.
Why count from the LMP and not conception?
Because the LMP is the date people actually know. Gestational age runs from it, which gives the count a two-week head start over the embryo — a “17-week” pregnancy conceived about 15 weeks ago. The convention is universal, and the page keeps it.
How does the cycle adjustment work?
One day per day of difference from 28: a 24-day cycle moves the due date 4 days earlier, a 33-day cycle 5 days later. It is a stated convention for regular cycles inside the 21-to-35 band — not a correction for irregular ones, which belong with a clinician.
How accurate is the date?
About 4% of babies arrive on it; the honest window is 38 to 42 weeks, with 37 and 42 the clinical edges of term. First-trimester ultrasound dates more accurately than the LMP method — when the two disagree, clinical practice revises to the scan.
Why 280 days?
Forty weeks of gestation counted the gestational way — from the LMP. It is the centre of the distribution, not a countdown with a fuse: the page prints the window beside the date so the estimate reads as an estimate.
What if my cycle is irregular?
The 21-to-35 adjustment assumes a regular rhythm; irregular cycles make the LMP method itself shaky, and dating ultrasound is the honest ruler. The page refuses cycle lengths outside the band rather than printing a false precision.
Does the page handle leap years?
Yes — the 280-day walk runs on the real calendar, leap days included. A 29th of February inside the span shifts nothing manually; the date arithmetic crosses it exactly.
What is the difference between this and the pregnancy-week page?
Direction. This page runs the LMP FORWARD to the 280-day line and prints the date; that page reads WHERE YOU ARE now — weeks, days, trimester, progress. Same convention, same LMP, opposite questions.