An estimated due date, from the first day of the last period.
The standard estimate is Naegele’s rule: LMP + 280 days (40 weeks), which assumes a 28-day cycle with ovulation on day 14. Only about 1 in 20 babies arrive on the exact date — the due date is the center of a normal window of roughly 37–42 weeks. A dating ultrasound, especially in the first trimester, refines it and overrides this estimate when they disagree. This calculator is information, not medical advice.
How accurate is the due date? It’s an estimate with a normal range of weeks around it. Your care provider’s dating (usually ultrasound-based) is the one to plan around.
My cycle isn’t 28 days. Does that change it? Yes — longer cycles shift the date later, shorter cycles earlier, roughly day-for-day. Mention your usual cycle length at your first appointment.