Biorhythm Calculator
Biorhythm theory says three smooth cycles start on the day you are born and repeat for life. Enter your birth date to chart them around any day, and read the history and the evidence below.
For entertainment and self-reflection: Biorhythm theory has been tested many times and has not been supported by research; the chart is a curiosity and a reflection prompt, not a guide to safety or performance. Nothing on this page is a prediction, and it is not financial, medical, psychological or legal advice. Use it as a prompt for thought, not as a basis for decisions.
Chart your biorhythms
Chart shows 15 days either side of the selected date. Red = physical, blue = emotional, green = intellectual. The dashed line is the selected day.
How biorhythms are calculated
Classic biorhythm theory uses three cycles, each a sine wave that starts at zero and rising on the day you are born:
- Physical: 23 days, said to govern strength, stamina and coordination.
- Emotional: 28 days, said to govern mood and sensitivity.
- Intellectual: 33 days, said to govern memory, alertness and reasoning.
For any date, count the days t since birth. The value of each cycle is sin(2π × t ÷ period), shown here as a percentage from −100% to +100%. Positive values were said to be "high" phases, negative values "low" or recharging phases. The days on which a curve crosses zero were called critical days, supposedly the least stable. A cycle crosses zero twice per period: on the first day of the cycle and halfway through. With an odd period, such as 23 days, the halfway crossing falls in the middle of a day (day 11.5), and the calculator marks the day in which the crossing happens.
The calculator counts whole calendar days between the two dates, so time zones and birth times do not matter. Because 23, 28 and 33 have no common factor, the three cycles only line up again after 23 × 28 × 33 = 21,252 days, about 58 years and 2 months.
Worked example
Someone born on June 15, 1990 looks at October 24, 2026. That is 13,280 days later.
- Physical: 13,280 ÷ 23 leaves a remainder of 9, so they are 9 days into the 23-day cycle. sin(2π × 9 ÷ 23) ≈ +63%, past the peak and falling.
- Emotional: the remainder after dividing by 28 is 8, so 8 days into the cycle. sin(2π × 8 ÷ 28) ≈ +97%, just past the top of the curve.
- Intellectual: the remainder after dividing by 33 is 14, so 14 days into the cycle. sin(2π × 14 ÷ 33) ≈ +46%, falling toward its next zero crossing around day 16.5.
In the language of biorhythm charts, that would be a good day all round. The calculator shows these numbers, a chart of the month around the date, and the critical days coming up.
Why the three cycles drift in and out of step
Because the three periods are different, the curves keep sliding past each other. The physical and emotional cycles, for example, line up again every 644 days (23 × 28), about 21 months, while the emotional and intellectual cycles realign every 924 days. Days when all three curves are high together, or all low together, are therefore fairly rare, and biorhythm fans used to look out for them. You can see this on the chart: move the date forward a week at a time and watch the three colours drift apart and come back together.
This is the same arithmetic that governs gears, planetary alignments and calendar cycles such as the 52-year Calendar Round of the Maya and Aztecs, which is one reason biorhythms are a popular classroom example. The cycles are perfectly regular, so a biorhythm chart for any date, past or future, can be computed exactly, with no measurement involved at all.
History: from Vienna to the 1970s craze
The idea goes back to the end of the nineteenth century. The Berlin physician Wilhelm Fliess, a friend and correspondent of Sigmund Freud, proposed that human life followed a 23-day "male" cycle and a 28-day "female" cycle. At about the same time the Viennese psychologist Hermann Swoboda described similar periods. A 33-day intellectual cycle was added later, usually credited to Alfred Teltscher, an Austrian engineering teacher, in the 1920s.
Biorhythms became a popular craze in the 1970s, helped by George Thommen’s book Is This Your Day? and by the arrival of pocket calculators and home computers, which made charts easy to produce. Coin-operated biorhythm machines appeared in shopping centres, and some companies tried using biorhythms to schedule work.
What the research says
Because the theory makes clear predictions (accidents and poor performance should cluster on critical days), it was easy to test, and it was tested many times in the 1970s and 1980s, on everything from industrial accidents to sports results and exam scores. A comprehensive 1998 review by Terence Hines in Psychological Reports looked at more than 130 studies and concluded that the theory is not valid: studies that found support generally had methodological problems, while well-designed studies found no effect. Scientists also point out that real body rhythms, such as the daily circadian cycle or the menstrual cycle, vary from person to person and respond to light and other cues, while biorhythm cycles are fixed from birth and identical for everyone.
That is why this calculator sits in our Fun & Culture section. It is an interesting piece of twentieth-century popular science, and the mathematics is a neat illustration of how cycles with different periods drift in and out of step.
A sensible way to use it
- As a curiosity. Compare your chart with how you actually feel for a few weeks. Most people find no consistent link, which is itself an interesting little experiment in noticing bias.
- As a maths example. Biorhythms are a friendly way to explain sine waves, periods and least common multiples to students.
- As a reminder to track the real thing. If you want to understand your energy, track sleep, exercise and mood directly. Those patterns are real and you can act on them.
- Never for safety. Do not use biorhythms to decide whether to drive, operate machinery, exercise hard or take a medical decision.
Frequently asked questions
How are biorhythms calculated?
Count the days since birth. Each cycle’s value is the sine of 2π times the day count divided by its period: 23 days for physical, 28 for emotional and 33 for intellectual.
What is a critical day?
A day on which one of the curves crosses zero, moving from positive to negative or back. Biorhythm theory claimed these were unstable days, but research has not supported this.
Do biorhythms work?
Studies have not supported them. A 1998 review of more than 130 studies concluded that biorhythm theory is not valid. Enjoy the chart as a curiosity.
Who invented biorhythms?
The 23- and 28-day cycles were proposed by Wilhelm Fliess and Hermann Swoboda around 1900. The 33-day intellectual cycle was added in the 1920s, usually credited to Alfred Teltscher.
When do all three cycles start again together?
After 21,252 days (23 × 28 × 33), which is about 58 years and 2 months after birth.
Does my birth time matter?
No. Classic biorhythm charts count whole days, so only the birth date is needed.
Sources and further reading
- Hines, T. M. "Comprehensive review of biorhythm theory", Psychological Reports 83 (1998): 19–64.
- Thommen, G. S. Is This Your Day?, Crown, 1964.
- Encyclopaedia Britannica, "Biorhythm".
- Wikipedia: "Biorhythm (pseudoscience)".
- Fliess, W. Der Ablauf des Lebens, Deuticke, 1906.
By Stanley King · About 8calc · Last reviewed October 2026