Biorhythm Calculator (Physical, Emotional & Intellectual Cycles)
Calculate your biorhythm for any date from your birth date: the physical (23-day), emotional (28-day), intellectual (33-day) and intuitive (38-day) cycles as percentages, rising or falling, critical days and a 14-day chart.
How it works
n = days since birth · physical = sin(2π·n ÷ 23) × 100% · emotional = sin(2π·n ÷ 28) × 100% · intellectual = sin(2π·n ÷ 33) × 100% · intuitive = sin(2π·n ÷ 38) × 100% · rising when cos(2π·n ÷ P) > 0; a "critical day" is when a curve crosses zero
Biorhythm theory, proposed by Wilhelm Fliess around 1900 and popularised in the 1970s, claims that three sine waves starting at birth govern physical strength (23 days), mood (28 days) and thinking (33 days), with "critical days" when a curve crosses zero. The calculation is simple trigonometry and is shown exactly. The theory itself, however, has been tested repeatedly — including in studies of accidents, sports results and exam scores — and none found any predictive power, so treat the numbers as a fun curiosity, not a planning tool.
Worked example
Someone born May 15, 1990 is 13,291 days old on October 4, 2026. 13,291 ÷ 23 leaves 20 days into the physical cycle: sin(2π × 20/23) = -73%, rising — it crosses zero (a critical day) on October 7. The emotional cycle is at -90% and falling (19 of 28 days), and the intellectual cycle is at its low point, -100%, and about to rise (25 of 33 days).
Sources: Hines, "Comprehensive review of biorhythm theory", Psychological Reports (1998) · Encyclopaedia Britannica — Biorhythm
Frequently asked questions
Are biorhythms real?
No reliable evidence supports them. Dozens of studies (reviewed by Hines in 1998) found no link between biorhythm cycles and accidents, illness, performance or mood. The calculator is offered for fun.
What is a critical day?
In biorhythm theory, a day when a cycle crosses the zero line between its positive and negative halves — said to be unstable. It happens roughly every 11.5 days for the physical cycle, 14 for the emotional and 16.5 for the intellectual.
When do all three cycles line up again?
The three main cycles return to their starting point together every 23 × 28 × 33 = 21,252 days, about 58 years and 67 days after birth.
Why is the emotional cycle 28 days?
Fliess linked it to the menstrual cycle and the lunar month, and the 23-day physical cycle to a supposed "male" rhythm. The 33-day intellectual cycle was added later by Alfred Teltscher. None of these periods has a physiological basis.
Related calculators
For entertainment and cultural interest only. Astrology and numerology have no scientifically demonstrated power to describe personality or predict events.