Pregnancy Due Date Calculator
Estimate a due date from your last period, adjusted for your actual cycle length rather than an assumed 28 days.
Pregnancy Due Date Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Assumes regular, predictable ovulation and an accurately recalled period date.
- Cannot account for irregular cycles, recent contraception, or breastfeeding.
- A first-trimester ultrasound is materially more accurate and overrules this estimate.
In short: A last period of 6 July 2026 on a 30-day cycle gives an estimated due date of 14 April 2027 — two days later than unadjusted Naegele's rule. About 4% of babies arrive on the estimated date and roughly 70% within a week either side.
Formula
The 280 days assume a 28-day cycle. Every day your cycle differs from 28 moves the due date by exactly one day.
Worked Example
- Start from the last period. 6 July 2026. Pregnancy is dated from here because it is the one date that can actually be observed.
- Apply Naegele's rule. 6 July 2026 + 280 days = 12 April 2027, assuming a 28-day cycle.
- Adjust for the real cycle. A 30-day cycle is 2 days longer than assumed, so add 2 days: 14 April 2027.
- Locate conception. Ovulation is cycle day 30 − 14 = day 16, so conception is about 22 July 2026 — 266 days before the due date.
- Read the window, not the date. Full term runs 7 to 20 April 2027 — 39w0d to 40w6d around the ADJUSTED date. Only ~4% arrive on the estimated date itself.
The cycle adjustment is the part most due date calculators skip, and it is not a rounding detail. Naegele's rule assumes ovulation on day 14 of a 28-day cycle, so on this person's 30-day cycle it places conception two days too early and the due date with it. On a 35-day cycle the error reaches a full week, and on a 21-day cycle it runs a week in the other direction. Since the same figure drives decisions about induction and post-term monitoring, a week is not trivial. The honest framing is that the adjusted date is a better centre for a wide distribution — and that a first-trimester scan beats both, which is why it overrules them.
Strengths & Limits Of This Model
Where this engine is strong
- Adjusts for actual cycle length instead of assuming 28 days.
- Shows the full term windows rather than a single date.
- States the real distribution of births, including the 4% figure.
Where it stops
- Cannot date an IVF pregnancy from transfer without the transfer date.
- Cannot replace ultrasound dating.
Practical Use Cases
Getting a realistic date before your first scan
The cycle-adjusted figure is the best you can do from a remembered period. A scan will refine it.
Understanding why your scan date differs
A discrepancy over 7 days in the first trimester means the scan wins. That is guideline, not opinion.
Planning leave around a window rather than a day
Full term spans two weeks. Plan for the window, not the date.
Checking the fertile timing behind the date
The same cycle arithmetic drives the Ovulation Calculator.
Methodology & Editorial Standards
A due date is estimated by adding 280 days to the first day of the last menstrual period, a convention named after Franz Naegele and in use since the early nineteenth century. The convention embeds two assumptions: that the cycle is 28 days long and that ovulation occurs on day 14. This engine keeps the classic figure visible but defaults to a cycle-adjusted date, adding the difference between the stated cycle length and 28, because a cycle two days longer delays ovulation by two days and moves the entire pregnancy with it. Conception is placed at cycle length less luteal phase, since the luteal phase is the near-constant part of the cycle, and the resulting interval from conception to due date is 266 days regardless of cycle length — the biological duration, as distinct from the 280-day convention dated from a period that precedes conception by roughly two weeks. Term windows follow the standard obstetric definitions, with early term at 37 to 38 completed weeks, full term at 39 to 40, late term at 41 and post-term from 42. All date arithmetic is performed in UTC so that daylight-saving transitions cannot shift a result by a day. The engine states the empirical distribution of births explicitly, because a single date implies a precision that the underlying biology does not support. This calculator is an educational estimate and is not medical advice, diagnosis or treatment. Cycle-based predictions assume regular ovulation and cannot account for irregular cycles, hormonal contraception, recent pregnancy or breastfeeding, thyroid or polycystic ovary syndrome, perimenopause, or any medication affecting the cycle. Dates produced here must not be used as contraception, and they do not establish paternity or any legal fact. A first-trimester ultrasound dates a pregnancy far more accurately than any calculation from a remembered period, and it takes precedence. Discuss results with a midwife or doctor. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
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.
Pregnancy Due Date Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How is a due date calculated?
Last period plus 280 days, adjusted for cycle length. Here 6 July 2026 on a 30-day cycle gives 14 April 2027.
What is Naegele's rule?
Add 280 days to the first day of the last period. It assumes a 28-day cycle with ovulation on day 14.
How many babies are born on their due date?
About 4%. Roughly 70% arrive within a week either side, and the full distribution spans about five weeks.
Why adjust for cycle length?
A longer cycle means later ovulation. On a 35-day cycle unadjusted Naegele is a full week early.
Which is more accurate, my dates or the scan?
The scan. First-trimester ultrasound dates to 5 to 7 days, and it takes precedence when the two differ by more than 7 days.
What counts as full term?
39 weeks 0 days to 40 weeks 6 days. Early term is 37 to 38+6, late term 41 weeks, post-term from 42 weeks.
Is 41 weeks overdue?
No. It sits inside the normal range and about 6% of births happen then. The word overdue causes needless anxiety.
How is an IVF due date calculated?
From the transfer: 261 days for a day-5 blastocyst, 263 for a day-3 embryo. It is the most precise dating there is.
Why is pregnancy dated from my last period?
Because that date is observable while conception is not. It adds about two weeks before conception to the official count.
How long is pregnancy from conception?
About 266 days, against the 280-day convention from the last period. Both describe the same pregnancy.
What if I do not remember my last period?
Ask for an early scan. Only about half of women recall the date accurately, which is the weakest link in the calculation.
Can a due date change?
Yes, usually once, after a first-trimester scan. After the first trimester it should rarely move.
Does a long cycle mean a longer pregnancy?
No. It means later ovulation, so the pregnancy starts later. Its actual length is unchanged at about 266 days.
What if my cycles are irregular?
Cycle-based dating becomes unreliable. An early scan is the only dependable method in that case.
Does this work for twins?
The dating arithmetic is the same, but twins deliver earlier on average and are managed to a different timetable.
Should I plan around the due date?
Plan around the full term window instead. Two weeks of uncertainty is normal and expected.
Is this pregnancy due date calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.