Menstrual Cycle Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Method from Office on Womens Health: Your menstrual cycle. Last verified 2026-06-23.

The Menstrual Cycle Calculator computes menstrual cycle from the relation estimated ovulation day = cycle length - luteal phase length. It takes 2 inputs (average cycle length in days, luteal phase length in days) and returns the menstrual cycle. 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 average cycle length = 30 days, luteal phase length = 14 days, the menstrual cycle works out to 16, 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 Office on Womens Health: Your menstrual cycle, 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.