Biorhythm Calculator by date of birth

What are biorhythms and what do the three cycles mean?

Biorhythms, in the sense used by a biorhythm calculator, are three regular cycles that are said to begin at birth and repeat at fixed lengths for life: a physical cycle of 23 days, an emotional cycle of 28 days and an intellectual cycle of 33 days. Each cycle is drawn as a smooth wave that rises from zero to a high point, falls back through zero to a low point and returns to zero. In the classic theory the physical cycle is associated with strength, stamina and coordination, the emotional cycle with mood and sensitivity, and the intellectual cycle with reasoning, memory and concentration. These associations describe what the tradition's followers believed each curve reflected.

The cycle lengths are fixed and identical for every person; nothing about your lifestyle, health or location changes them. The only thing that differs from person to person is the date of birth, so two people born on the same day have exactly the same biorhythm chart for life. Because the three lengths differ, the curves drift in and out of alignment, and all three return to the birth position together only after 23 × 28 × 33 = 21,252 days, roughly 58.2 years.

Days are counted from the start of each period.

How is a biorhythm calculated from your date of birth?

A biorhythm is calculated with a single sine function: value = sin(2π × t / P), where t is the number of whole days since your birth and P is the length of the cycle in days (23, 28 or 33). The result lies between −1 and +1, and the calculator multiplies it by 100 to show a percentage from −100% to +100%. Nothing else enters the calculation: no time of birth, no place, no personal details.

Take a worked example with 10,000 days since birth. For the physical cycle, 10,000 divided by 23 leaves a remainder of 18, so sin(2π × 18 / 23) ≈ −0.98, a value of about −98%, close to the bottom of the cycle. For the emotional cycle the remainder of 10,000 divided by 28 is 4, and sin(2π × 4 / 28) ≈ +0.78, or +78%. For the intellectual cycle the remainder divided by 33 is 1, giving sin(2π × 1 / 33) ≈ +0.19, or +19%. Only the remainder matters.

Leap days count as ordinary days: two people born on 1 March 1991 and 1 March 1992 are 366 days apart, because 1992 was a leap year, and the calculator counts calendar days for you. The birth day itself is day zero, when all three values are sin(0) = 0. The theory never defined an hour of birth or a time zone, so two calculators can disagree by a day if one counts the birth day as day one; such a shift moves every critical day by one day and changes nothing else.

What are critical days in biorhythm theory?

Critical days in biorhythm theory are the days on which a cycle curve crosses the zero line, either falling from the positive half of the cycle into the negative half or rising back. A sine wave crosses zero twice per period, so the physical cycle has a critical day every 11.5 days, the emotional cycle every 14 days and the intellectual cycle every 16.5 days. The tradition treated these transition days as unsettled and advised extra care on them.

The day of birth is by definition a critical day in all three cycles at once, since every curve starts at zero. A day when two curves cross zero together is called a double critical day. A triple critical day, when all three cross at once, requires the day count to be a multiple of 11.5, 14 and 16.5 at the same time, which happens only every 10,626 days, about every 29.1 years.

One property of the emotional cycle follows from arithmetic alone. Its length of 28 days is exactly four weeks, so its critical days always fall on the weekday of your birth: if you were born on a Tuesday, every emotional critical day of your life is a Tuesday. The 23-day and 33-day cycles have no such link with the week. With half-day periods such as 11.5 and 16.5 days, the exact crossing often falls between two calendar days, which is why many traditional charts describe a critical period of about three days rather than a single date.

How do you read a biorhythm chart?

A biorhythm chart is read from left to right like a calendar: the horizontal axis is the date, the vertical axis is the cycle value from −100% at the bottom to +100% at the top, and three coloured lines show the physical, emotional and intellectual cycles. A 30-day chart is long enough to show at least one peak, one trough and one zero crossing for each cycle.

The upper half of the chart is called the positive or high phase in the tradition, the lower half the negative or low phase. The peak of a cycle comes a quarter of the way through it and the trough at three quarters. Traditional readings interpreted a high phase as a time when the quality of that cycle was freely available and a low phase as a time for rest. These are interpretations from the tradition; the research summarised further down has not confirmed them.

The most informative feature of the chart is the pattern across the three curves rather than any single value. Days when all three are high, all three are low, or all three cross zero at once are the ones that followers of the theory paid most attention to. Because the lengths are 23, 28 and 33 days, such coincidences are uncommon, which is part of what made the charts feel meaningful.

Where did biorhythm theory come from?

Biorhythm theory came from Berlin in the late nineteenth century, when the physician Wilhelm Fliess proposed that human life is governed by a 23-day cycle and a 28-day cycle counted from birth. Fliess was a close correspondent of Sigmund Freud, and his periodic numbers appear throughout their letters. He called the 23-day period the male cycle and the 28-day period the female cycle and believed both were present in everyone.

At about the same time the Vienna psychologist Hermann Swoboda wrote about similar cycles, which he said he had observed in his patients. The third cycle appeared in the 1920s, when Alfred Teltscher, a teacher in Innsbruck, reported a 33-day pattern in his students' academic performance and proposed it as an intellectual cycle. With that addition the theory took the three-cycle form that biorhythm calculators still use today.

The theory reached a mass audience much later. George Thommen's book Is This Your Day? (1964) presented biorhythms as a system anyone could chart, and in the 1970s Bernard Gittelson's Biorhythm: A Personal Science made the word widely known. Pocket biorhythm calculators were sold in the 1970s. That popularity is also what prompted researchers to test the theory systematically.

What are the extra biorhythm cycles: intuition, aesthetic, awareness and spiritual?

The extra biorhythm cycles are four longer periods added after the classic three: an intuition cycle of 38 days, an aesthetic cycle of 43 days, an awareness cycle of 48 days and a spiritual cycle of 53 days. They are later additions with no connection to Fliess, Swoboda or Teltscher, and their names vary between publications. The classic theory has only three cycles, and the studies reviewed by Hines concern those three.

The four extra lengths continue the arithmetic pattern of the original cycles: each is five days longer than the last, so the full sequence runs 23, 28, 33, 38, 43, 48, 53. The calculator computes them with the same sine formula, so the 53-day spiritual cycle, for example, peaks on day 13.25 and crosses zero every 26.5 days. Their names describe what their authors believed each cycle governed, and, as with the classic cycles, these descriptions belong to the tradition.

How does biorhythm compatibility between two people work?

Biorhythm compatibility compares the same cycle in two people and measures how closely their curves move together. Because every person's cycles start at birth and have the same fixed lengths, the only thing that separates two people's curves is the number of days between their birthdays. If that difference is an exact multiple of the cycle length, the curves are identical. If it is exactly half a cycle, the curves are mirror images, and one person is at a peak whenever the other is at a trough.

The calculator expresses this as a percentage for each cycle with the formula |cos(π × (d mod P) / P)| × 100, where d is the difference in days between the two birth dates and P is the cycle length. Any multiple of 23 days gives 100% for the physical cycle, and a quarter cycle gives about 71%. For the 28-day emotional cycle a difference of 14 days gives exactly 0%. For the two odd-length cycles the lowest possible score is slightly above zero, about 7% for the physical and 5% for the intellectual cycle, because a whole-day difference can never land exactly on the half cycle.

A pair born exactly one ordinary year apart is 365 days apart, one day past a multiple of 28, and scores about 99% on the emotional cycle, about 92% on the physical cycle (365 mod 23 = 20) and about 98% on the intellectual cycle (365 mod 33 = 2). Two people can just as easily be closely matched on one cycle and out of phase on another. The score describes the geometry of two sine waves and nothing more; the research reviewed below found no evidence that the cycles predict behaviour, so a percentage between two people is best read as a curiosity of the tradition.

How accurate are biorhythm charts according to research?

Biorhythm charts are not accurate as predictions: the most thorough review of the evidence, published by the psychologist Terence Hines in Psychological Reports in 1998 (volume 83, issue 1, pages 19 to 64), examined 134 studies and concluded that biorhythm theory is not supported. The studies had checked whether accidents, sports results and other measurable events clustered on critical days or in low phases. In properly controlled studies, events fell on critical days no more often than chance would predict.

Why, then, did the theory feel convincing to so many people? Part of the answer lies in how it is set up. With three cycles crossing zero every 11.5, 14 and 16.5 days, roughly one day in five is critical in at least one cycle, and if the day on either side is counted as well, more than half of all days are covered. Any accident or bad day therefore has a good chance of landing near a critical day. Add the habit of remembering hits and forgetting misses, and a chart that is wrong in general can still feel right in particular.

This is why this page describes biorhythms as a tradition and a tool for self-observation. The chart is an exact mathematical object that follows precisely from your birth date and the formula; what the research shows is that the curves do not forecast how a day will go.

Are biorhythms the same as circadian rhythms?

No, biorhythms and circadian rhythms are different things, and the similarity of the names causes constant confusion. Circadian rhythms are biological cycles of roughly 24 hours that run in the cells of almost every living organism: the sleep-wake cycle, daily swings in body temperature and the timed release of hormones such as melatonin are all circadian. They are driven by molecular clocks inside cells and reset every day by light. In 2017 the Nobel Prize in Physiology or Medicine went to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for discovering the molecular mechanisms that control the circadian rhythm.

Biorhythms, by contrast, are multi-day cycles of 23, 28 and 33 days counted from the date of birth. No biological clock with those periods has been identified, the cycles do not respond to light, sleep or any other input, and the review described above found no predictive value in them. When you read that scientists have confirmed the existence of biological rhythms, the research concerns circadian and related rhythms, never the three biorhythm curves.

How can you use a biorhythm chart today?

A biorhythm chart can be used today the way its twentieth-century followers used it: as a fixed calendar to compare with your own notes. The chart depends only on your birth date, so it can be drawn years ahead and never changes, which makes it easy to test for yourself. Keep a short daily record of how you felt or what you got done, then look back after a month or two and check whether your notes line up with the curves.

A few practical points make the comparison fair. Write your notes before you look at the chart, so the curve does not colour your memory of the day. Record the misses as carefully as the hits, since a critical day that passes without incident is evidence too. And count how many days were critical in at least one cycle; seeing how much of the calendar the theory covers is often the most instructive result.

The compatibility percentages for two people stay the same for life, because they depend only on the gap between the two birthdays. As a conversation piece or a curiosity, that is exactly what the tradition offers. As a basis for decisions about relationships, work or health, it goes beyond anything the evidence supports.