Naegele’s rule calculator, with working and practice

Enter the LMP to get the expected date of delivery with every step of Naegele’s rule shown — +7 days, −3 months, +1 year — the way it’s written in nursing and medical exams. Then test yourself with unlimited practice questions.

Naegele’s rule formula

EDD = LMP + 7 days − 3 months + 1 year

Equivalently, add 9 months and 7 days. The rule comes from the average pregnancy length of 280 days (40 weeks) from the first day of the last period, assuming a 28-day cycle with ovulation on day 14. The background is explained in Naegele’s rule and your due date.

Worked examples

Notice the second and third examples: when the +7 days rolls into the next month, you subtract 3 months from the new month. Adding 9 months and 7 days in one go gives the same answer; it’s quickest when the LMP falls in January to March, because the EDD is then in the same year.

Practice questions

Fresh questions every time. Choose a type, answer, and press Check to see the full working.

Common mistakes

• Using the last day of the period. The LMP is always the first day of bleeding.
• Forgetting the month roll-over. 27 May + 7 days is 3 June, so subtracting 3 months gives 3 March, not 3 February.
• Forgetting the year. Subtracting 3 months goes back in time; you must then add 1 year.
• Counting weeks from conception. POG and gestational age count from the LMP, about 2 weeks before conception — see gestational age vs. fetal age.
• Ignoring a longer cycle. Add the extra days beyond 28 (e.g. +4 days for a 32-day cycle).

When the scan overrides the LMP

Naegele’s rule is the starting estimate. Under ACOG guidance, a first-trimester ultrasound measuring the crown-rump length is used to confirm it, and the due date is changed if the two differ by more than 5 days before 9 weeks, or by more than 7 days between 9 weeks and 13 weeks 6 days. For a full revision guide, see EDD calculation for nursing students, or print a paper OB wheel.

Frequently asked questions

What is the Naegele’s rule formula?

EDD = first day of LMP + 7 days − 3 months + 1 year. For example, an LMP of 10 June 2026 gives 17 June, then 17 March, then 17 March 2027. It is the same as adding 9 months and 7 days.

Why does Naegele’s rule sometimes differ from adding 280 days?

Because calendar months have different lengths. The 7-days-minus-3-months shortcut can land a day or two away from an exact 280-day count. Both are accepted in practice; exam answers usually expect the Naegele working.

How do I adjust Naegele’s rule for a longer cycle?

The rule assumes a 28-day cycle. A common adjustment is to add the extra days for a longer cycle (or subtract for a shorter one): a 32-day cycle moves the EDD 4 days later.

How do I calculate the period of gestation (POG)?

Count the days from the first day of the LMP to the date in question, divide by 7, and write the result as completed weeks plus remaining days — for example 92 days = 13 weeks 1 day, written 13+1.

Is Naegele’s rule still used?

Yes, as the starting estimate from the LMP. ACOG recommends confirming it with a first-trimester ultrasound, and changing the due date if the scan differs by more than 5 days before 9 weeks or more than 7 days from 9 to 13 weeks 6 days.

Sources

Last meaningfully updated: 2026-10-20.