How to Calculate Your Due Date (Naegele's Rule Explained)
Your due date is estimated by adding 280 days, 40 weeks, to the first day of your last menstrual period (LMP). This is Naegele’s rule, and if your average cycle isn’t exactly 28 days, the estimate shifts by the difference. That’s the whole calculation. The rest of this article walks through why it works this way, how much a cycle length actually moves the date, and where the method breaks down.
Naegele’s rule, explained properly
Naegele’s rule is named after the 19th-century German obstetrician Franz Karl Naegele, and it is still the default method doctors and midwives use for a first due date estimate. The formula is:
Due date = first day of last menstrual period + 280 days
Two things make this formula work, and both are approximations baked in on purpose.
First, a full-term pregnancy is counted as 280 days, 40 weeks, from LMP, even though you weren’t actually pregnant for the first two weeks of that count. Conception happens around ovulation, not on day one of your period. The method counts from LMP anyway because it’s the one date most people can pin down with confidence; nobody tracks the exact day an egg was fertilized, but most people know when their last period started.
Second, the 280-day figure assumes ovulation happens on day 14 of a 28-day cycle, roughly two weeks before the next period would have started. If your average cycle runs longer or shorter than 28 days, ovulation shifts along with it, and so does conception. A 35-day cycle means you likely ovulate around day 21, not day 14, so the whole pregnancy timeline slides later by that same difference. The adjustment is simple: take your actual average cycle length, subtract 28, and add that many days (positive or negative) to the 280-day baseline.
So the fuller formula is:
Due date = LMP + 280 days + (cycle length − 28)
A 32-day cycle adds 4 extra days to the estimate. A 24-day cycle subtracts 4. This single adjustment is the difference between a due date that’s roughly right and one that’s off by a week for no reason other than an unquestioned assumption about cycle length.
Worked example
Take a last menstrual period starting January 1, with an average cycle length of 28 days, the textbook case.
| Metric | Value |
|---|---|
| Last menstrual period (LMP) | January 1 |
| Cycle length | 28 days |
| Days added | 280 |
| Estimated due date | October 8 |
| Estimated conception date | around January 15 |
The arithmetic: January has 31 days, so 280 days from January 1 lands 30 days into the rest of January, then all of February through September, then into October. Count it out month by month and day 281 of the year (LMP counted as day 1) falls on October 8, which is why the tool’s own example uses exactly this pair of dates. Conception is estimated 14 days after LMP, around January 15, since that’s when ovulation is assumed to happen in a 28-day cycle.
How cycle length shifts the due date
Here’s the part most explanations skip: the same LMP produces a different due date depending on your actual cycle length. Using January 1 as LMP for all four rows below, and applying the (cycle length − 28) adjustment directly to the baseline of October 8:
| Cycle length | Adjustment | Estimated due date |
|---|---|---|
| 24 days | 4 days earlier | October 4 |
| 28 days (average) | baseline | October 8 |
| 32 days | 4 days later | October 12 |
| 35 days | 7 days later | October 15 |
Between a 24-day and a 35-day cycle, the estimate for the exact same LMP moves by 11 days, more than a week and a half. That gap is why entering your real cycle length matters more than most people assume; defaulting to 28 when your actual average is 32 or 35 quietly pushes your estimated due date too early.
Three ways a due date actually gets estimated
LMP and Naegele’s rule is only one of three methods used in practice, and they don’t always agree.
LMP / Naegele’s rule. What this tool calculates. Fast, requires only two pieces of information (LMP date and cycle length), and is the standard first estimate given at an early prenatal visit. Its accuracy depends entirely on regular cycles and a confidently remembered LMP date, which is exactly where it tends to fail.
Ultrasound dating scan. An early ultrasound, usually between 8 and 13 weeks, measures the fetus’s crown-rump length and compares it against known growth curves to estimate gestational age directly. This is generally considered more accurate than LMP dating, particularly for irregular cycles, because it measures the actual size of the fetus rather than assuming a textbook ovulation day. When an ultrasound estimate and an LMP estimate disagree by more than about a week, clinical guidelines typically favor the ultrasound date.
IVF transfer date. For pregnancies from in vitro fertilization, the due date is calculated from the known embryo transfer date, since the exact age of the embryo at transfer (day 3 or day 5, typically) is known precisely. This is the most accurate of the three methods, because it removes the guesswork around ovulation timing entirely; conception timing isn’t estimated, it’s a matter of clinical record.
Calculate your own due date
Enter the first day of your last period and your actual average cycle length below. The result updates as you type and also shows your estimated conception date and how far along you are today.
Common mistakes and edge cases
Assuming a 28-day cycle by default. As the table above shows, using 28 when your real average is 32 or 35 shifts the due date by four to seven days on its own. If you don’t know your average cycle length offhand, it’s worth checking a few months of tracking before trusting the estimate.
Treating the due date as a guarantee. A due date is the middle of a range, not a deadline. Only about 1 in 20 babies is born on the exact estimated date; most arrive anywhere from a week before to two weeks after. Plans built around the exact day, rather than a window around it, tend to cause unnecessary stress when the date passes without labor starting.
Using a conception date as if it were an LMP. The two methods aren’t interchangeable without adjustment. If you know your conception date (from ovulation tracking or a fertility monitor), the equivalent LMP is roughly two weeks earlier, not the conception date itself. Plugging a conception date directly into an LMP-based calculator overstates how far along you are by about two weeks.
Irregular cycles undermining the whole method. Naegele’s rule assumes a consistent cycle length and a clearly known LMP. If your cycles vary widely month to month, or you’re unsure exactly when your last period started, the LMP estimate becomes unreliable in a way no cycle-length adjustment can fix. An early ultrasound dating scan is the right next step in that case, and it’s what most care providers will recommend anyway.
This calculation is an estimate for planning purposes. It does not replace prenatal care, and any due date, however it’s calculated, should be confirmed and tracked with a doctor or midwife.
Frequently asked questions
How accurate is a due date calculated with Naegele’s rule? It gives a reasonable estimate for a pregnancy with regular cycles, but it’s a statistical middle point, not a prediction. Only about 1 in 20 babies is born on the exact calculated date, and the normal range of full-term birth spans several weeks around it. An early ultrasound is generally more accurate, especially outside of a textbook 28-day cycle.
What should I do if my cycle is irregular? Enter your best average cycle length if you have one, but treat the result as a rough starting point rather than a firm date. Irregular cycles make the ovulation-day assumption behind Naegele’s rule unreliable, and a dating ultrasound in the first trimester should take priority over the LMP-based estimate once it’s available.
Can this tool calculate a due date from a conception date or IVF transfer date instead of LMP? Not directly. This calculator works from the first day of your last menstrual period. If you know your conception date, you can approximate an equivalent LMP by subtracting about two weeks and enter that instead. For IVF pregnancies, the due date is usually calculated straight from the embryo transfer date using a fixed offset for the embryo’s age at transfer, which is a separate calculation from the LMP method this tool uses.
Why does the due date change when I adjust the cycle length field? Because the tool applies the same adjustment described above: the difference between your entered cycle length and 28 days is added to or subtracted from the 280-day baseline. A longer cycle means later ovulation, so the due date moves later; a shorter cycle moves it earlier. It’s the same arithmetic shown in the cycle-length table earlier in this article, just calculated live for whatever date and cycle length you enter.