Naegele's Rule Due Date Calculator
Naegele's rule is the long-standing method for estimating a due date from the first day of the last menstrual period (LMP). It adds 280 days (40 weeks) to the LMP, which assumes a regular 28-day cycle. Because ovulation timing shifts with cycle length, this calculator lets you adjust for cycles shorter or longer than 28 days. It returns your estimated date of delivery (EDD), the conception-based estimate, and your current gestational age in completed weeks. The due date is always an estimate and should be confirmed by your healthcare provider, ideally with early ultrasound dating.
Naegele's rule formula
EDD = LMP + 280 days + (cycle length - 28)
Conception date = LMP + 14 days + (cycle length - 28)
Gestational age = (today - LMP) / 7 weeks
Days to due date = EDD - today
The classic mnemonic is: take the LMP, add one year, subtract three months, add seven days. That yields the same 280-day result for a 28-day cycle. The cycle adjustment shifts ovulation, and therefore the EDD, by the difference between your cycle length and 28 days.
Due date context
- Full-term pregnancy averages 40 weeks (280 days) from the first day of the LMP.
- Only about 4 percent of babies arrive on the exact estimated due date.
- Most births occur within two weeks before or after the EDD.
- Early ultrasound dating is generally more accurate than LMP dating, especially with irregular cycles.
- Term is defined as 37 to 42 completed weeks of gestation.
Naegele's rule: frequently asked questions
What is Naegele's rule?
Naegele's rule is a standard method for estimating the estimated date of delivery (EDD). It takes the first day of the last menstrual period (LMP), adds one year, subtracts three months, and adds seven days. This is equivalent to adding 280 days (40 weeks) to the LMP, assuming a regular 28-day cycle with ovulation on day 14.
Why 280 days?
A typical full-term pregnancy is about 40 weeks measured from the first day of the last menstrual period, which is 280 days. Ovulation and conception occur roughly two weeks after the LMP, so gestation from conception is about 266 days (38 weeks).
How does cycle length change the estimate?
Naegele's rule assumes a 28-day cycle. If your cycle is longer or shorter, ovulation shifts. This calculator adjusts the due date by adding (cycle length minus 28) days, so a 32-day cycle pushes the EDD 4 days later and a 26-day cycle moves it 2 days earlier.
Is the due date guaranteed?
No. The EDD is an estimate. Only about 4 percent of births occur on the exact due date. Most births occur within two weeks before or after. An early ultrasound is generally more accurate than LMP dating, especially when the LMP is uncertain or cycles are irregular.
Does this replace medical care?
No. This tool is for general information only. Your due date should be confirmed by a qualified healthcare provider, who may use ultrasound dating. Always follow the guidance of your obstetric care team.
Official sources
- American College of Obstetricians and Gynecologists: Clinical guidance.
- U.S. National Library of Medicine, StatPearls: Gestational Age Assessment.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.