By Biorhythms
Biorhythm compatibility measures how closely two people's three cycles line up — physical (23 days), emotional (28 days) and intellectual (33 days). The score depends only on the difference between the birth dates, so it doesn't change over time.
How it's calculated
Biorhythm theory claims that from the moment of birth, three sine waves with constant periods run through a person's life. For a couple, the phase difference of each one is calculated.
If the difference between the birth dates is a multiple of the period, the phases coincide completely — 100%. If the difference equals half a period, the phases are opposite — 0%.
The percentage for each cycle is the share of days when that cycle is in its high phase (above zero) for both people at once, or in its low phase (below zero) for both at once.
Since the difference between two birth dates is constant, a couple's "biorhythm compatibility" is the same on every day of their lives. Sites that show it changing from day to day are calculating something else.
Worked Example
February 14, 1988 and July 8, 1990 are 875 days apart.
875 ÷ 23 = 38 cycles and 1 day — the physical rhythms almost coincide, about 91%.
875 ÷ 28 = 31 cycles and 7 days — the emotional rhythms are shifted by a quarter period, 50%.
875 ÷ 33 = 26 cycles and 17 days, so the next alignment is 16 days away — the intellectual rhythms are almost in antiphase, about 3%.
The average score is about 48%. Notice how different the values of the three cycles are: "overall biorhythm compatibility" is the average of three loosely related numbers.
What the result means
According to the theory, matching physical rhythms mean a shared pattern of activity and rest, matching emotional rhythms mean matching moods, and matching intellectual rhythms mean similar periods of mental clarity.
In practice, something else matters more: biorhythm theory has been tested many times since the 1970s, and no link has been found between these cycles and actual performance, accident rates or mood. Reviews of dozens of studies reached the same conclusion — there is no effect.
That's why we present the calculation as arithmetic, not as a recommendation.
Table of Meanings
| Metric | Period | What the theory attributes to it |
|---|---|---|
| Physical | 23 days | strength, stamina, daily rhythm |
| Emotional | 28 days | mood, sensitivity |
| Intellectual | 33 days | clarity of thought, memory |
You can view a chart of your own biorhythms on a separate page, which also covers the history of the theory and why it wasn't confirmed.
Frequently asked questions
Why doesn't the result change from day to day?
Because it isn't supposed to: what's calculated is the phase difference between two people, and that is constant. Each person's biorhythms change, but not their position relative to each other.
Where do the 23-, 28- and 33-day periods come from?
The first two were proposed by Wilhelm Fliess in the late 19th century; the third was added in the 1920s. These numbers have no physiological basis whatsoever.
Should I rely on this?
No. It's the only method in this section that is known for certain to have been tested and not confirmed.
Then why have a calculator?
Because the arithmetic here is honest and verifiable, and people do ask for it. We do the calculation and tell you straight away what the number is worth.