Biorhythm calculator
Three cycles of 23, 28 and 33 days, counted from the day you were born.
Other things worked out from a birth date: birth chart, horoscope calculator, rising sign, moon sign, big three, numerology calculator, angel number calculator, Chinese zodiac calculator, birth date calendar, lunar calendar and personal year.
Does this work? No.
The 23- and 28-day cycles were proposed in the 1890s by Wilhelm Fliess, a Berlin physician and friend of Freud; the 33-day intellectual cycle was added in the 1920s by the Austrian engineer Alfred Teltscher. The idea was popularised from the 1960s by George Thommen’s book Is This Your Day? and then by pocket calculators and coin-operated biorhythm machines in the 1970s. 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. 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.
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 — each cycle crosses zero roughly every two weeks, so almost any date is within a few days of one.
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.