Naegele's Rule Due Date Calculator
Naegele's rule is the classic way doctors and midwives work out a due date from the first day of your last period. Enter that date and your cycle length, and this calculator shows every step of the rule, your due date, how far along you are and your full-term window.
What is Naegele's rule?
Naegele's rule is a simple method for estimating a baby's due date. It is named after Franz Naegele, a German obstetrician who described it in the early 1800s, and it is still taught to doctors, midwives and nurses today. The rule assumes a pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period (LMP) and that ovulation happens on day 14 of a 28-day cycle.
Naegele's rule formula
Due date = first day of LMP + 7 days − 3 months + 1 year
Subtracting 3 months and adding 1 year is the same as adding 9 months, so you can also think of it as LMP + 9 months + 7 days.
Example: if your last period started on January 10, 2026:
| Step | Result |
|---|---|
| Add 7 days | January 17, 2026 |
| Subtract 3 months | October 17, 2025 |
| Add 1 year | October 17, 2026 |
Adjusting for longer or shorter cycles
The original rule works best for a regular 28-day cycle. If your cycle is longer, you probably ovulated later, so your due date moves later too. A common adjustment, known as Parikh's rule, adds or subtracts the difference between your cycle length and 28 days:
Adjusted due date = Naegele's date + (cycle length − 28) days
For example, with a 32-day cycle you add 4 days; with a 25-day cycle you subtract 3 days. This calculator makes that adjustment for you.
Naegele's rule vs counting 280 days
Some calculators simply count 280 days from your last period. Because months have 28 to 31 days, this can land one to three days away from the Naegele date. Neither method is "wrong". Both are estimates, and the difference is tiny compared with the natural spread of birth dates.
How accurate is Naegele's rule?
Only about 4 to 5% of babies are born on their exact due date. Most arrive within about two weeks either side of it. The rule is less accurate if:
- your cycles are irregular or you are not sure of your period date
- you recently stopped hormonal birth control or were breastfeeding
- you had bleeding in early pregnancy that looked like a period
For these reasons, an ultrasound in the first trimester is the most accurate way to date a pregnancy. If the scan date differs from your LMP date by more than about 5 to 7 days, doctors usually go with the scan.
Frequently asked questions
How do you calculate a due date with Naegele's rule?
Take the first day of your last period, add 7 days, subtract 3 months and add 1 year. For example, an LMP of March 1 gives a due date of December 8.
Why does Naegele's rule add 7 days?
Adding 9 months to your LMP gives about 273 days. The extra 7 days bring the total to about 280 days, the average length of pregnancy counted from the last period.
Does Naegele's rule work for irregular periods?
It is less reliable. If your cycles are irregular, an early ultrasound gives a better date. If you know your conception date, the Due Date Calculator can use that instead.
Is my due date the day I will give birth?
Usually not. It is the midpoint of the likely range. Babies born between 37 and 42 weeks are considered term.
Can my due date change?
Yes. Your doctor may update it after your first ultrasound. Once a due date is set by an early scan, it is not normally changed by later scans.