How it works
Estimate an expected delivery date from the first day of your last menstrual period using Naegele's rule.
Enter the first day of your last menstrual period (LMP) and, optionally, your cycle length. The calculator adds 280 days (adjusted for cycle length) to estimate the due date and reports the approximate conception date.
Formula
Due date = LMP + 280 + (cycle length − 28) days. Approximate conception = LMP + 14 days.
LMP- First day of last menstrual period (date)
cycle- Menstrual cycle length (days)
Examples
- LMP 1 January 2023, 28-day cycleLMP 2023-01-01 · cycle 28 daysEstimated due date 2023-10-08 · conception ≈ 2023-01-15
Frequently asked questions
How reliable is the due date?
Only about a small percentage of babies arrive on the estimated date. It sets a 40-week reference point, not a guaranteed delivery date.
What if my cycle is not 28 days?
Enter your average cycle length. The estimate is adjusted by (cycle length − 28) days to account for a longer or shorter cycle.
How to read the result
The estimate assumes a 28-day cycle and conception around day 14. Individual timing varies, so treat the result as a planning estimate.
Limitations
Naegele's rule is a standard estimate, not medical advice. Confirm dates with a healthcare provider, who can refine the estimate with ultrasound.
Source: Naegele's rule (LMP + 280 days, cycle-adjusted), the standard obstetric dating method.