Reverse Due Date Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Method from MedlinePlus Prenatal Care. Last verified 2026-06-23.

The Reverse Due Date Calculator computes reverse due date from the relation days from today to LMP = (due date serial day - 280) - today serial day. It takes 6 inputs (due date year, due date month 1 to 12, due date day 1 to 31, today year, today month 1 to 12, today day 1 to 31) and returns the reverse due date. Because this is a pure mathematical or physical formula rather than a jurisdiction-specific rule, the result never changes over time: the same inputs always produce the same answer, so you can rely on it whether you are checking homework, sizing a design, or sanity-checking another tool. Enter your values in the fields below and the result updates instantly; you can also share a permalink that pre-fills the exact calculation, which is useful for teaching, reports, or collaboration. For example, with due date year = 2026, due date month 1 to 12 = 10, due date day 1 to 31 = 8, today year = 2025, today month 1 to 12 = 12, today day 1 to 31 = 1, the reverse due date works out to 31, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. The method is the standard form documented by MedlinePlus Prenatal Care, and the figure above each result carries the date it was last verified. This tool is general information and is not a substitute for professional engineering, medical, financial, or scientific advice; always check critical results against the primary source and your own judgement.