Biorhythm Calculator

Biorhythm theory proposes that human performance fluctuates in predictable sine-wave cycles from the moment of birth. The three primary cycles are Physical (23-day) for stamina and coordination, Emotional (28-day) for mood and sensitivity, and Intellectual (33-day) for analytical thinking and memory. Extended models add secondary cycles: Intuitive (38-day), Aesthetic (43-day), and Spiritual (53-day). At birth each cycle starts at zero, rises to a peak, falls through zero to a trough, and repeats indefinitely.

No. Multiple peer-reviewed studies have failed to confirm predictive validity for biorhythm cycles. The theory is not accepted by mainstream science or medicine. Originated by Wilhelm Fliess and Hermann Swoboda in the late 19th century, it peaked in pop-culture popularity in the 1970s. Use this tool for self-reflection and entertainment — not for medical, professional, or safety-critical decisions.

Each cycle value uses the formula: sin(2π × days_since_birth ÷ cycle_length) × 100. The result ranges from −100 (trough) to +100 (peak). Days since birth is the number of complete days elapsed between your birth date and the target date. Example for Physical: sin(2π × 10,000 ÷ 23) × 100. All calculations happen entirely in your browser — no data is sent to any server.

Critical days occur when a biorhythm cycle crosses the zero line — transitioning from positive to negative or vice versa. Biorhythm practitioners consider these transition days to be periods of heightened unpredictability where that cycle's energy is unstable or unreliable. For the Physical cycle they happen roughly every 11.5 days; for Emotional every 14 days; for Intellectual every 16.5 days. When multiple cycles are critical on the same day, some practitioners consider it especially significant.

Compatibility mode overlays two people's biorhythm cycles on the same chart and calculates alignment scores. A score close to 100% means both people are in similar phases (e.g., both near peak) while a low score means they are in opposite phases. The alignment per cycle is calculated as: 100 − (|person1_value − person2_value| ÷ 2). The overall score averages across the primary three cycles.