Due Date Calculator
Due date from the last menstrual period.
Estimated due date
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Enter the first day of your last period.
What is a due date?
The estimated due date is calculated from the last menstrual period (LMP) plus the average pregnancy length. Actual delivery may vary by 1-2 weeks, so this is a reference only — confirm with an obstetric ultrasound.
How it is calculated (Naegele's rule)
Naegele's rule adds 280 days (40 weeks) to the last menstrual period.
Due date = LMP + 280 days
Current pregnancy week = (today − LMP) in days ÷ 7
The logic behind Naegele's rule — why 280 days
Naegele's rule is a 19th-century rule of thumb attributed to German obstetrician Franz Karl Naegele. It estimates the due date by adding 280 days to the last menstrual period (LMP) — equivalent to subtracting three months and adding seven days.
It assumes ovulation typically occurs around the midpoint of the cycle, about day 14 of a 28-day cycle, and adding the average gestational length from conception works out to roughly 280 days (40 weeks) when counted from the LMP instead. This calculator's result follows the same standard assumption.
The limits of Naegele's rule — why it's a reference only
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your actual cycle is shorter, longer, or irregular, the gap between the calculated due date and your real ovulation and conception timing can grow larger.
Actual delivery also rarely lands on the exact estimated day — it commonly happens within a range of weeks before or after it — so the due date is best understood as the center of an average estimated range rather than a fixed day. Early-pregnancy ultrasound measurements of fetal size are generally considered more precise than an LMP-based estimate, so always confirm your exact pregnancy dating and due date through consultation with an obstetrician.