About this tool
Biorhythm Calculator is a free, in-browser tool that plots the three classic biorhythm cycles for any date from your birthday. Nothing is uploaded — the cycles are computed locally from just two dates.
It counts the whole days between your birth date and the target date, then evaluates three sine waves that are assumed to start at birth: the physical cycle with a 23-day period, the emotional cycle with 28 days, and the intellectual cycle with 33 days. Each value is sin(2π × days ÷ period) scaled to a percentage from −100% to +100%, and an arrow shows whether the cycle is rising (↑) or falling (↓) into the next day. Zero-crossings are the so-called 'critical' days in biorhythm lore.
Biorhythm theory is a pseudoscience with no scientific validity — it has been repeatedly tested and does not predict mood, energy or performance. Treat this as a curiosity. Set your birth date and a target date (today by default) and the three percentages and days-alive count update instantly.
Frequently asked questions
What do the three cycles and periods mean?
Biorhythm theory assigns three cycles that all start at birth: physical (23-day period), emotional (28-day) and intellectual (33-day). Each is a sine wave sampled at the number of days you have been alive.
How is each percentage calculated?
For a cycle of length P, the value is sin(2π × daysAlive ÷ P) × 100, giving a figure between −100% and +100%. Days alive is the whole number of days from your birth date to the target date. The arrow compares today's value with the next day's to show rising or falling.
Is there any science behind biorhythms?
No. Biorhythm theory is a well-known pseudoscience; controlled studies have found it has no predictive power. This calculator is for entertainment only and should never guide health, safety or important decisions.
Is my birth date uploaded anywhere?
No. The two dates stay in your browser and the sine-wave maths runs entirely on your device — nothing is sent to a server or stored.
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