Biorhythm Calculator

Biorhythm theory proposes that human physical, emotional, and intellectual states follow predictable sinusoidal cycles from birth. Though not recognized by mainstream science, biorhythms remain a popular tool for personal planning and self-awareness.

The three core biorhythm formulas: Physical(d) = sin(2π × d / 23) Emotional(d) = sin(2π × d / 28) Intellectual(d) = sin(2π × d / 33)

Where:

• d = number of days elapsed since birth date
• 23 = physical cycle period (days), governs stamina, coordination, and strength
• 28 = emotional cycle period (days), governs mood, creativity, and emotional sensitivity
• 33 = intellectual cycle period (days), governs analytical thinking, memory, and decision-making

Each formula produces values between -1.0 (lowest point) and +1.0 (peak). A value of 0 is called a critical day, a transition point associated with instability in that dimension.

Results below are shown as percentages, so a sine value of 0.94 prints as 94%.

Cycle phases, as this calculator labels them:

The critical band is checked first, so a value of 3% reads as Critical rather than Neutral. That ordering matters: a zero crossing sitting inside the neutral band is the one day the theory says to pay attention to, and it would otherwise be labeled as the least interesting day of the month.

Critical days come in pairs. Each cycle crosses zero twice, once going up and once coming down, so they arrive every half period: every 11.5 days for the physical cycle, 14 for the emotional, 16.5 for the intellectual. Two of those three periods are odd numbers, which is why a crossing often falls in the middle of a day rather than at midnight.

Worked example: A person born January 15, 1990, checking their biorhythms on April 8, 2026: Days elapsed = 13,232 Physical = sin(2π × 13,232 / 23) = sin(3,614.8 rad) ≈ +0.94 (high) Emotional = sin(2π × 13,232 / 28) = sin(2,969.4 rad) ≈ -0.43 (low) Intellectual = sin(2π × 13,232 / 33) = sin(2,519.4 rad) ≈ -0.19 (mildly low)

Composite score: average of all three = (0.94 - 0.43 - 0.19) / 3 ≈ +0.11, a moderately stable day overall.

Treat the composite gently. Averaging three cycles of different lengths hides more than it shows, because a +94% physical day paired with a -43% emotional day is not the same experience as three flat cycles, and both average out near zero. The individual figures are the interesting part. Best used as a rough self-reflection guide, not a deterministic forecast.