Biorhythm calculator
This biorhythm calculator counts the days since you were born and places you in three cycles: physical (23 days), emotional (28) and intellectual (33). The arithmetic is exact; the theory behind it has failed every serious test.
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Does this work? No.
Biorhythm theory claims three fixed cycles running from birth, identical for everyone and never varying with illness, age or circumstance. It was assembled in stages:
- 1890s — Wilhelm Fliess, a Berlin physician and friend of Freud, proposes the 23- and 28-day cycles.
- 1920s — the Austrian engineer Alfred Teltscher adds the 33-day intellectual cycle.
- 1960s — George Thommen’s book Is This Your Day? brings the idea to a wide audience.
- 1970s — pocket calculators and coin-operated biorhythm machines make it a fad.
It has been tested extensively — against industrial accidents, aviation incidents, athletic performance and exam results. Terence Hines’s 1998 review of more than a hundred studies in Psychological Reports found no support for the theory, and the studies that seemed to support it had methodological problems.
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.
How the cycles are calculated
Count the days from birth to the date in question. Each cycle is a sine wave: the physical value is sin(2π × days ÷ 23), the emotional value sin(2π × days ÷ 28) and the intellectual value sin(2π × days ÷ 33), each shown here as a percentage from −100% to +100%. Every cycle starts at zero on the day you were born and rises first.
All three start at zero on the day of birth and drift apart at once. By day 99 they sit at three different points, and they only line up at zero again after 21,252 days.
Days when a cycle crosses zero are called critical days and were said to be the riskiest. Because 23, 28 and 33 share no common factor, all three cycles return to zero together only once every 21,252 days — about 58 years and two months after birth. The emotional cycle, at exactly four weeks, crosses zero on the same weekday you were born, every time.
Why it looked convincing
Everyone has good and bad days, and a chart with three waves always has one of them rising or falling. Looking back at an accident and finding a "critical day" nearby is easy, because critical days are everywhere: by this calculator's own rule, 13.9% of all days are critical for at least one cycle, and 57% of days fall within two days of one.
0 means the day itself is critical. No day is ever more than a week from a critical day, so a "critical day near the accident" is close to guaranteed.
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.
Cite this page
AstroHot. “Biorhythm Calculator: Physical, Emotional, Intellectual.” Accessed November 1, 2026. https://astrohot.com/biorhythm-calculator/
For Wikipedia
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