Free due date calculator
Get your estimated due date in one tap. Enter the first day of your last period and the calculator counts forward 280 days with Naegele's rule to return your due date — the centre of your 40-week pregnancy, updated live, as you type.
On this page12 sections
Due date is an estimate based on average 40-week gestation. Always confirm with your OB/GYN or midwife via ultrasound.
Results are estimates. Consult a professional.
What is a due date?
Your estimated due date (EDD) is the date your baby is predicted to arrive — 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). It is the single date that anchors your whole pregnancy: it sets how far along you are, when each prenatal test is scheduled, and the point from which doctors judge whether a baby is early, on time, or late. This due date calculator returns that date the moment you enter your LMP.
It is worth saying plainly: a due date is an estimate, not an appointment. Babies arrive across a range of weeks, and the EDD simply marks the centre of that range. Roughly 1 in 20 babies is born on the exact due date — the rest arrive in the weeks around it.
How is a due date calculated?
The standard method is Naegele's rule, named after the 19th-century German obstetrician Franz Naegele. It adds 280 days — exactly 40 weeks — to the first day of your last period. The calculator above uses this rule directly: enter your LMP and it counts forward 280 days.
Why 280 days when pregnancy is ~38 weeks?
A pregnancy lasts about 38 weeks from conception, but the due date is counted from your last period — roughly two weeks earlier, because ovulation and conception happen about 14 days into the cycle. Those two extra weeks are why the count is 40 weeks (280 days) rather than 38. It also means that at the moment of conception you are already considered "2 weeks pregnant" by this dating method.
A worked example using the due date calculator
Aisha's last period began on January 1, 2026. She enters that one date into the calculator. Here is the arithmetic it runs — and the same result worked by hand using both forms of Naegele's rule.
- Enter the LMP. Aisha types January 1, 2026 into the single date field — the only input the calculator needs.
- The +280-day form. The calculator adds 280 days to January 1, 2026. That lands on October 8, 2026 — her estimated due date.
- Check it the calendar way. By hand: take LMP minus 3 months (January 1 → October 1), add 7 days (→ October 8), add 1 year (→ October 8, 2026). Both forms agree.
- Read it as a window. Full term is 39–40 weeks, so Aisha can reasonably expect labour any time from late September to mid-October — with October 8 as the centre point, not a guarantee.
Dating from LMP, ultrasound, conception, or IVF
There is more than one way to date a pregnancy, and they are not equally precise. This calculator uses the LMP / Naegele method because it needs only a date you already know. Clinics refine that estimate with an early ultrasound, and people who conceived through IVF can date from the transfer almost to the day.
| Dating method | What it uses | Typical accuracy |
|---|---|---|
| LMP (Naegele's rule) | First day of last period + 280 days | ± about 2 weeks; this calculator's method |
| First-trimester ultrasound | Crown-rump length (CRL) measurement | ± about 5–7 days — the most accurate |
| Known conception / ovulation date | Conception + 266 days (38 weeks) | Tight if ovulation was confirmed |
| IVF embryo transfer | Transfer date + embryo age | Near-exact — the conception date is known |
ACOG treats first-trimester ultrasound as the most reliable dating tool; LMP is the everyday estimate this calculator provides.
How accurate is a due date?
A due date is a well-grounded prediction, not a deadline. Only about 4% of babies are born on their exact due date. The vast majority arrive in the two weeks on either side of it, which is why doctors treat anything from 39 to 41 weeks as normal timing rather than early or late.
| When babies arrive | Roughly how common |
|---|---|
| On the exact due date | About 4% |
| Within 1 week either side | The majority of births |
| Before 37 weeks (preterm) | About 1 in 10 |
| At or after 42 weeks (post-term) | Uncommon — most are induced first |
Birth-timing spread is why the due date is best read as the centre of a ±2-week window, not a fixed day.
Accuracy also depends on which method set the date. An LMP-based estimate is good to about ±2 weeks; a first-trimester ultrasound narrows that to roughly ±5–7 days. The further into pregnancy an ultrasound is done, the less accurate it becomes for dating — a baby's size varies more between individuals later on, so a third-trimester scan can be off by three weeks or more. That is why the very first scan is the one that fixes your date.
Why babies rarely come on the due date
Two things conspire against the exact date. First, the 280-day count assumes a textbook 28-day cycle with ovulation on day 14 — but real cycles and ovulation timing vary, so the "40 weeks" is itself an average. Second, the length of a healthy pregnancy genuinely varies from one person to the next, by as much as five weeks even when dating is precise. The due date captures the middle of that natural spread, which is exactly why so few births land on it. First-time mothers, in particular, tend to deliver a few days after their due date on average.
Full-term, preterm, and post-term — what the weeks mean
"Term" is not a single week. In 2013, ACOG and partner bodies split the weeks around the due date into named bands, because outcomes differ even between 37 and 39 weeks. Knowing these labels makes a due date far easier to read.
| Term category | Gestational age | Relative to a 40-week due date |
|---|---|---|
| Preterm | Before 37 0/7 weeks | More than 3 weeks early |
| Early term | 37 0/7 – 38 6/7 weeks | 1–3 weeks early |
| Full term | 39 0/7 – 40 6/7 weeks | The week of and just before the due date |
| Late term | 41 0/7 – 41 6/7 weeks | Up to 1 week late |
| Post-term | 42 0/7 weeks and beyond | 2+ weeks late |
Source: ACOG Committee Opinion 579, Definition of Term Pregnancy (2013).
These bands replaced the older, blunter idea that anything from 37 to 42 weeks was simply "term." The change matters because outcomes improve across those weeks: babies born in the early-term window (37–38 weeks) have measurably higher rates of breathing and feeding difficulty than those born at full term (39–40 weeks). That is why elective deliveries are generally not scheduled before 39 weeks without a medical reason.
What can change your due date?
The calculator returns one date from your LMP, but several real-world factors can move the clinical estimate — almost always at your first ultrasound. The most common reasons a due date shifts:
- An early ultrasound disagrees. If a first-trimester scan dates the pregnancy more than 7 days off your LMP estimate, the scan date wins.
- Irregular or long cycles. Naegele's rule assumes a 28-day cycle with day-14 ovulation; longer cycles push the true due date later than the LMP suggests.
- An uncertain LMP. If you don't remember the exact first day of your last period, or had bleeding in early pregnancy, the LMP estimate is less reliable.
- Conception via IVF. The transfer date is known precisely, so an IVF due date is calculated from the embryo's age rather than an LMP.
- Hormonal contraception before conceiving. Cycles can take time to regularise after stopping the pill, making a recent LMP a shakier anchor.
Data sources and methodology
The due date is calculated with Naegele's rule — LMP plus 280 days (40 weeks) — the method recommended by the American College of Obstetricians and Gynecologists in Committee Opinion 700, "Methods for Estimating the Due Date". The term-category weeks (early, full, late, and post-term) follow ACOG Committee Opinion 579, "Definition of Term Pregnancy" (2013). The dating-accuracy figures — ultrasound to about ±5–7 days, LMP to about ±2 weeks, and the 7-day redating threshold — reflect ACOG and NHS guidance. All dates here are computed with plain calendar arithmetic from the LMP you enter.
ACOG — Methods for Estimating the Due Date (Committee Opinion 700).NHS — Pregnancy due date calculator.Frequently asked questions about the free due date calculator
About this due date calculator
This due date calculator runs entirely in your browser. The date you enter never leaves your device — nothing is sent to a server, logged, or shared. It applies Naegele's rule (last period + 280 days) to estimate a 40-week due date and updates instantly on every change.
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