Biorhythm calculator
Three cycles of 23, 28 and 33 days, counted from the day you were born.
Does this work? No.
Biorhythm theory was proposed around 1900 by Wilhelm Fliess, a Berlin physician and friend of Freud, and popularised in the 1970s when pocket calculators made it easy. It claims three fixed cycles running from birth, identical for everyone, never varying with illness, age or circumstance.
It has been tested extensively — against industrial accidents, aviation incidents, athletic performance and exam results. A 1978 review by Terence Hines covering dozens of studies found no support whatsoever, and later work has not changed that.
The calculator above is exact. What it calculates has no demonstrated relationship to anything. That is worth saying plainly on the page rather than in a disclaimer at the bottom.
The real thing it gets wrong
Human biology does run on cycles — circadian rhythm on roughly 24 hours, the menstrual cycle on roughly 28 days, seasonal patterns across a year. These are measurable, they respond to light, sleep, stress and age, and they differ between people.
Biorhythm theory takes that real observation and replaces it with three fixed numbers that never change for anyone. The idea that a cycle starts at birth and runs unaltered for eighty years is the part with no biological basis at all.