Obstetricians have used the same basic formula for generations to give expectant parents an initial due date estimate. Our pregnancy due date calculator applies that formula — Naegele’s rule — plus shows key trimester milestone dates along the way.
The formula
Due date = first day of last menstrual period (LMP) + 280 days (40 weeks)This assumes a textbook 28-day cycle with ovulation on day 14 — a simplification that doesn’t precisely match every individual’s actual cycle length.
Example
LMP of January 1, 2026:
January 1 + 280 days = October 8, 2026 (estimated due date)Trimester milestones
Alongside the due date, the calculator shows estimated end-of-trimester dates (day 91 for end of first trimester, day 182 for end of second trimester, day 280 for full term) — useful reference points for prenatal appointment scheduling.
Important limitations
Only about 5% of babies are actually born on their exact calculated due date — full-term birth is considered entirely normal anywhere from 37 to 42 weeks. If your menstrual cycle isn’t close to 28 days, your actual due date may differ meaningfully from this calculation. Healthcare providers typically refine the estimate using an early ultrasound, which can be more accurate than LMP-based dating alone, especially for people with irregular cycles.
This tool is a convenient starting reference, not a medical diagnosis — always follow your healthcare provider’s official due date.