Biorhythm Calculator

Enter your date of birth to chart your physical, emotional and intellectual cycles around today. Free, instant, and everything stays on your device.

Physical (23 days) Emotional (28 days) Intellectual (33 days)
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Physical
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Emotional
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Intellectual
An honest note: biorhythm theory — the idea that fixed 23, 28 and 33-day cycles run from birth — is not supported by scientific evidence. Controlled studies have found no reliable link between these cycles and real performance, mood or health. It is included here purely as a bit of fun and self-reflection, not as a tool for planning, health or decisions.
For reflection and entertainment only. This is not medical, psychological or predictive advice, and the curves are a simple mathematical pattern rather than a measurement of you.

How to use the Biorhythm Calculator

  1. Enter your date of birth in the first field.
  2. Optionally choose a date to centre the chart on — it defaults to today.
  3. Click "Chart my biorhythms" to draw the three cycles for the 15 days before and after that date.
  4. Read today's percentages, the high/low states and any critical days flagged where a cycle crosses zero.

Why use ZillaKit's Biorhythm Calculator?

Biorhythm theory proposes that from the moment of birth, three cycles rise and fall like sine waves: a physical cycle of 23 days linked to energy and coordination, an emotional cycle of 28 days linked to mood and sensitivity, and an intellectual cycle of 33 days linked to focus and reasoning. This calculator counts the exact number of days since your birth date and plots each cycle as a smooth wave across a canvas chart covering the fifteen days on either side of your chosen date, marking today with a vertical line and reading off each cycle's value as a percentage from -100% to +100%. Days where a cycle crosses the zero line are traditionally called critical days, and they are flagged for you. Everything is calculated instantly and entirely in your browser using plain trigonometry — there is no signup, no account, and nothing is uploaded or stored. Importantly, we are upfront that biorhythm theory has been tested and is not scientifically supported, so the whole thing is offered as a playful lens for reflection rather than any kind of real forecast about your body or mind.

FAQ

What are the three biorhythm cycles?

A physical cycle of 23 days, an emotional cycle of 28 days and an intellectual cycle of 33 days, each modelled as a sine wave starting at zero on the day you were born.

What is a critical day?

A critical day is when a cycle crosses the zero line, moving between its positive and negative halves. In the theory these are seen as less stable days, though there is no scientific evidence for this.

Is biorhythm theory scientifically proven?

No. Careful studies have found no reliable connection between these fixed cycles and real performance or wellbeing. It is offered here purely for fun and reflection, not for health or planning.

How are the percentages calculated?

Each percentage is the sine of the number of days since birth scaled to the cycle length, so +100% is the peak, -100% is the trough and 0% is a crossing point.

Is my birth date stored anywhere?

No. All calculations happen locally in your browser using plain JavaScript. Nothing is uploaded, logged or saved.