Biorhythm calculator

Biorhythm theory, popular in the 1970s, says three cycles start at birth and repeat for life:

Cycle	Length	Supposedly linked to
Physical	23 days	strength, stamina, coordination
Emotional	28 days	mood, sensitivity, creativity
Intellectual	33 days	reasoning, memory, alertness

The calculator shows each cycle's value for the start date as a percentage from −100% to +100%, and draws all three as curves over the period you choose. The chart begins three days before the start date, marked with a dashed line, so you can see which way each curve is heading.

Each curve is a simple sine wave:

value = sin(2π × days since birth ÷ cycle length)

Say you were born on May 15, 1990, and the start date is November 13, 2026. That is 13,331 days.

• Physical: 13,331 ÷ 23 = 579.6 cycles, so you are 0.61 of the way into the current cycle; sin(2π × 0.61) ≈ −63%.
• Emotional: 13,331 ÷ 28 = 476.1 cycles; sin(2π × 0.11) ≈ +62%.
• Intellectual: 13,331 ÷ 33 = 403.97 cycles; sin(2π × 0.97) ≈ −19%.

The page also tells you how many days old you are on the start date.

In traditional biorhythm reading, the days when a curve crosses the zero line are called critical days, said to be times of instability for that cycle. The calculator marks a day as critical when the curve crosses zero between that day and the next, shows a label under any cycle that is critical on the start date, and lists all critical days in the chosen period with the cycle names.

Because 23, 28 and 33 have no common factor, all three cycles line up at zero together only once every 21,252 days, or about 58 years.

Controlled studies that looked for links between biorhythm cycles and accidents, sports performance or exam results did not find any beyond chance. The curves depend only on how many days have passed since your birth; they don't measure anything about your body or mind. Real biological rhythms, such as the roughly 24-hour circadian cycle, are a separate, well-studied topic.

Use this calculator for fun or out of curiosity, and don't base decisions about health, driving, work or money on it.

• Compare two people by running the calculator twice with different birth dates; some people enjoy checking compatibility this way.
• The 60-day view shows almost two full intellectual cycles, which makes the pattern easy to see.
• Dates are entered in your browser's date picker and shown in MM/DD format on the chart axis.
• Nothing is stored or sent; the calculation runs on your device.

If you just want to know how many days old you are, the age calculator gives that directly.

If you track the chart for a few weeks out of curiosity, compare it honestly with how your days actually went. Most people find no consistent pattern, which matches what the research says.