Due Date Calculator
The formula
How accurate is a due date?
Only about 4–5% of babies arrive on their exact due date; the vast majority are born within two weeks either side, which is why “term” spans 37–42 weeks. LMP dating assumes ovulation on day 14 — irregular or longer cycles shift everything. A first-trimester ultrasound (crown-rump length) is more accurate than LMP and often revises the date; when the two disagree by more than ~5–7 days, clinicians use the ultrasound.
Frequently asked questions
What if I don’t know my LMP date?
Is 40 weeks from conception or LMP?
Source & methodology
Naegele’s rule (standard obstetric dating); ACOG guidance on term pregnancy (37–42 weeks) and first-trimester dating accuracy.
How Due Date works — and what your result really means
You are counting something that matters — this tool counts it exactly, including the edge cases calendars hide. There are plenty of pages about Due Date; almost none of them show the working. This one does: the formula, every input, a fully computed example, and a sensitivity analysis calculated live from this engine’s own defaults.
In plain terms, Due Date answers the question "Estimated due date?". The measurement matters because it converts a situation that feels complicated — several inputs pulling in different directions — into a single number you can act on. Within Miscellaneous & Fun, Dates & Time is the context where this figure is quoted most often, and professionals treat it as a first-pass screen rather than a verdict.
The genuine usefulness of light metrics in Dates & Time is that they teach the same habit the serious ones demand: define your inputs, check the arithmetic, question the assumptions.
Nothing below is stock text pasted around a number: the worked example computes estimated due date = October 17, 2026 from this tool’s own defaults. Read the sections in order and you will be able to reproduce every figure by hand.
The Due Date formula, explained
The engine implements a single stated model: EDD = LMP + 280 days + (cycle length − 28). There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.
First day of last menstrual period (example value —) is a plain measured quantity — the unit shown in the label is the unit the formula expects. If the period is partial, decide whether you mean elapsed or planned time and keep that meaning across every field.
Average cycle length (days) (example value 28) is a measured number in the unit named by the label; the formula does not convert for you. Keep every length field in the same unit; the formula assumes a consistency it cannot check for you.
Due Date on the example inputs: the complete substitution
The tool prints its own working; this is the same arithmetic on the example values, reproduced so you can follow it by hand. Starting figures: First day of last menstrual period, Average cycle length (days). The endpoint of that chain is October 17, 2026 of estimated due date, sitting in the “40 weeks from LMP (280 days)” band.
- Naegele’s rule: LMP + 280 days (40 weeks)
- Cycle adjustment: + (28 − 28) = +0 days
- Due date = October 17, 2026
Current gestational age — 40 weeks, 0 days. Compare it against the main result of October 17, 2026 before drawing conclusions.
Trimester — 3rd trimester. Read next to the headline, this is the figure that shows the shape of the trade-off.
Estimated conception date — January 24, 2026. If your inputs change, this is usually the first companion figure to move.
First ultrasound window (8 wks) — March 7, 2026. Read next to the headline, this is the figure that shows the shape of the trade-off.
Full term begins (37 wks) — September 26, 2026. This is the number experienced users quote when the headline alone would mislead.
Re-running with your own figures is instant; re-running by hand once is the best way to know the tool is telling you the truth.
Reading the result: bands, zones and thresholds
The value above only becomes useful the moment you know what it is being measured against. The verdict attached to the default calculation — “40 weeks from LMP (280 days)” — is one band on a ladder your own figure may sit somewhere else on. The practical reading rules: read the band as a starting hypothesis, then test it: change the two dominant inputs by ten percent and see whether the verdict survives. Verdicts that survive stress are the ones worth acting on.
Set Due Date beside its neighbours and it usually plays a specific role: this figure establishes the size of something, and the neighbouring tools establish whether that size is acceptable. Skipping the first step makes the second one arbitrary.
The errors that quietly break a Due Date figure
Chasing precision in Average cycle length (days) while First day of last menstrual period is still a guess. The sensitivity ranking above shows exactly where the accuracy budget should go.
Chasing precision in Average cycle length (days) while First day of last menstrual period is still a guess. The sensitivity ranking above shows exactly where the accuracy budget should go.
Another is comparing across contexts: a Due Date computed with one set of conventions should never be lined up against a figure computed with another. Check the inputs behind both before ranking them.
A quick check for any Due Date figure
- Recompute once with the dominant input deliberately set low, and see whether the decision changes.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Confirm the inputs are dated — a Due Date figure should carry the date it was computed.
- Compare the output against one published benchmark or a colleague’s independent estimate.
- Confirm the example values are plausible for your own situation before replacing them.
- Repeat the ±10 % test on the inputs that matter and see whether the decision — not just the number — still holds.
Step by step: getting a trustworthy Due Date out of this tool
The tool does the arithmetic; the routine below is what makes the arithmetic trustworthy. Five steps, a minute total — and afterwards you will know not just what the number is, but which assumption produced it.
Step 1 — start from evidence. The inputs behind Due Date are first day of last menstrual period, average cycle length (days). Write them down with their source next to each one; if you cannot name where a figure came from, that is the input most likely to be wrong. Say the answer out loud before you save it; if it sounds wrong, the model did not change, your understanding did.
Step 2 — enter the conservative case first. Resist the flattering scenario. Type the inputs you would defend in front of a sceptical reader, read the Estimated due date (October 17, 2026 for the example values), and note the verdict band. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 3 — play with the inputs on purpose. Ten percent up, ten percent down, one field at a time. Within a few minutes you will know this calculation better than most people who quote its results. The inputs are the part of the work that nobody else can do for you.
Step 4 — compute a corridor, not a point. Three runs — worst, likely, best — turn Due Date from a snapshot into a range. Plan against the pessimistic end and treat the optimistic end as the upside story. When two sources disagree, it is almost never the arithmetic that differs.
Step 5 — write the one-line memo. “Estimated due date = October 17, 2026 on this date, from these inputs.” That line is the entire deliverable; everything else on this page is how you earned the right to write it. Rounding early is the quietest way to introduce a permanent error.
From calculation to decision: where Due Date earns its keep
Miscellaneous & Fun arguments end at arithmetic or they do not end. Due Date is the arithmetic: October 17, 2026 on the example inputs, reproducible by anyone who types the same numbers. Repeating a calculation with the same numbers proves only that the arithmetic is reliable — which is worth knowing.
Used properly this is a five-minute filter that saves weeks. Compute Due Date, reject what fails, and only then bring in the expensive analysis for the survivors. The inputs are the part of the work that nobody else can do for you.
Treat Due Date like a smoke alarm rather than a painting — its value is in being tested periodically, not in being admired once. Precision is a promise you make about your data; make sure your inputs can keep it.
Due Date — frequently asked questions
What is a typical Due Date?
On the worked example above Due Date comes out at October 17, 2026. Treat that as one point on a distribution rather than a norm: the bands, the companion outputs and the sensitivity ranking tell you how wide the plausible range really is.
How do I calculate Due Date by hand?
Use the stated model — EDD = LMP + 280 days + (cycle length − 28) — and substitute in the order the tool prints (it starts from Naegele’s rule: LMP + 280 days (40 weeks)). On the example inputs the hand route lands on October 17, 2026, exactly what the calculator shows. A spreadsheet reproduces it in two or three cells; doing it once by hand is what makes the automated answer trustworthy.
Is the Due Date result of October 17, 2026 good or bad?
Good and bad are comparisons, so the honest test is three questions: where does your own Due Date sit against the October 17, 2026 reference above, how far would a plausible input error move it, and what does the alternative option compute to?
Is Due Date accurate?
The arithmetic is exact: EDD = LMP + 280 days + (cycle length − 28) is evaluated with full floating-point precision and every substitution is printed in the worked example. What limits accuracy is input quality and the model’s assumptions, so treat the October 17, 2026 result as the correct answer to the question you typed — and check the assumptions section before acting on it.
What affects Due Date the most?
Sensitivity depends on where you start, but the inputs First day of last menstrual period and Average cycle length (days) typically move Due Date further than the rest because they enter the formula multiplicatively. If you can only measure two things well, measure those and hold the others at conservative defaults.
What if I don’t know my LMP date?
An early ultrasound can date the pregnancy within about ±5 days in the first trimester. IVF pregnancies are dated from retrieval/transfer, not LMP.
Is 40 weeks from conception or LMP?
From LMP — which means conception typically occurs about 2 weeks into the “pregnancy” clock. Actual gestation is ~38 weeks.
Does this Due Date tool store my numbers?
No. The entire calculation runs in your browser session; nothing entered above is transmitted or saved, which is the correct behaviour for sensitive Miscellaneous & Fun figures and the reason you can re-run this page as often as you like.
The vocabulary of Due Date
- Current gestational age
- 40 weeks, 0 days on the example inputs — a companion figure printed beside the headline of Due Date so that no result is read in isolation.
- Trimester
- 3rd trimester for the Due Date worked case; it sits next to the main result because it carries context the headline leaves out.
- Estimated conception date
- January 24, 2026 on the example inputs — a companion figure printed beside the headline of Due Date so that no result is read in isolation.
- Estimated due date
- The primary output of this tool: October 17, 2026 for the default inputs, computed exactly from EDD = LMP + 280 days + (cycle length − 28).
- Worked example
- Every substitution in order, from the example inputs to October 17, 2026 — the audit trail for the headline figure.
- Verdict band
- A named zone on the range of plausible results; the example inputs land in “40 weeks from LMP (280 days)”, and your inputs may not.
Where the Due Date model comes from
Everything above runs on one stated formula, applied without hidden coefficients: EDD = LMP + 280 days + (cycle length − 28). Source of record: Naegele’s rule (standard obstetric dating); ACOG guidance on term pregnancy (37–42 weeks) and first-trimester dating accuracy. Specialist software layers extra coefficients on top of the same algebra; none are hidden here, and that transparency is what makes a disagreement traceable to a specific input instead of a black box.
This page is an estimation aid, not professional advice. The design rule is simple: two people entering identical figures must get identical answers. Nothing is smoothed, weighted or adjusted behind the scenes; where judgement enters, this page says so and names the input responsible.
Turning this Due Date into a decision
Re-run whenever a start date moves — time calculations compound small shifts into large misses.
The audience here is people planning around a point or span in time. Everything you need to defend the result — formula, substitution, sensitivity, source — is printed above, so the only remaining variable is the honesty of your inputs.