Pregnancy Due Date Calculator

Estimate a pregnancy due date from the last menstrual period using Naegele’s rule.

Due Date Calculator helps you estimate a due date from the first day of the last menstrual period (Naegele’s rule). Enter first day of last period, review the breakdown, and use the guidance below to understand what the number means — and what it does not. What the Due date calculator does Due Date Calculator is built for body measurements, calories, and everyday health math. Estimate a due date from the first day of the last menstrual period (Naegele’s rule). You enter first day of last period, then Simple Calculators computes the result instantly. These tools use widely published formulas. They are educational screens, not a diagnosis. How the math works Naegele’s rule: due date ≈ LMP + 280 days. Gestational age is days since LMP. The formula: Due date = first day of last period + 280 days. 280 days is 40 weeks. Assumes a 28-day cycle.