Four ways to calculate an EDD
| Method | Formula | Notes |
|---|---|---|
| Naegele’s rule | LMP + 1 year − 3 months + 7 days | Classic calendar method; assumes a 28-day cycle |
| 280-day rule | LMP + 280 days | Exact-day version used by pregnancy wheels and software |
| Cycle-adjusted | LMP + 280 days + (cycle − 28) | Allows for later or earlier ovulation in longer or shorter cycles |
| Parikh’s formula | LMP + 9 months + (cycle − 21) days | Calendar method that also adjusts for cycle length |
The calculator applies all four to your dates and shows how many days each differs from the cycle-adjusted result.
A worked example
Take a last period that began on May 15, 2026 and a 28-day cycle. Naegele’s rule: add a year (May 15, 2027), subtract three months (February 15, 2027), add seven days — February 22, 2027. Counting exactly 280 days instead gives February 19, 2027. The three-day gap appears because the months between May and February have more than 30 days on average. Parikh’s formula (+9 months + 7 days) also gives February 22.
That is why modern practice counts exact days: the 280-day rule is consistent whatever the month.
Who was Naegele?
Franz Naegele was a German obstetrician who popularised the rule in the early nineteenth century, based on earlier work. It has lasted because it is simple and works well enough for many pregnancies — but it was never meant to be exact, and today an early ultrasound often refines the date.
Which result should I use?
If your cycles are about 28 days, the 280-day result is the standard. If they are consistently longer or shorter, the cycle-adjusted date is usually closer. If you have had a dating scan, your provider’s date takes priority — the EDD Calculator Pregnancy shows how a scan and LMP dating are reconciled.