Biorhythm calculator
The cycles of 23, 28 and 33 days share no common factor, so they only return to their starting point together after 23 × 28 × 33 = 21,252 days — about 58 years and two months. That single date is the only thing about biorhythms that is mathematically interesting, and it is the reason the theory picked those particular numbers.
Biorhythm theory was invented in the late nineteenth century and has been tested repeatedly since. It has no predictive validity. This computes the sine waves the theory specifies.
Biorhythm theory proposes three sine waves running from birth: physical at 23 days, emotional at 28 and intellectual at 33. All three return to their starting alignment every 21,252 days, about 58 years.
How biorhythms are calculated
The theory dates from Wilhelm Fliess in the 1890s and enjoyed a revival in the 1970s, when it was tested seriously and repeatedly. Reviews of dozens of studies found no relationship between biorhythm phase and accidents, performance or anything else. The cycles here are computed exactly as the theory specifies, because that is what the page is for — but the honest framing is that this is a curiosity with a well-documented negative result behind it.
Questions
Physical at 23 days, emotional at 28 and intellectual at 33, each running as a sine wave from birth.