Biorhythm Calculator: Physical, Emotional, Intellectual
Calculate where your physical (23-day), emotional (28-day) and intellectual (33-day) cycles stand on a chosen date, based on Wilhelm Fliess’s biorhythm theory.
How biorhythms are calculated: the 23, 28 and 33-day formula
Each cycle equals sin(2π × days lived ÷ cycle length) × 100, with 23 days for the physical cycle, 28 for the emotional one and 33 for the intellectual one. Someone born on 15 May 1995 has lived 11,475 days on 14 October 2026, giving −52%, −90% and −99%. Biorhythm theory, put forward by Wilhelm Fliess in the early 20th century, has never been validated experimentally; the tool applies the formula with no predictive value.
Formula
Days lived = number of whole days between the date of birth and the analysis date. For each cycle of length T (23, 28 or 33 days): value = sin(2π × days ÷ T) × 100, rounded. The value ranges from −100% to +100% and crosses 0 every T ÷ 2 days, i.e. every 11.5 days for the physical cycle, 14 for the emotional one and 16.5 for the intellectual one. Biorhythm theory calls these zero crossings "critical days".
Example
Born 15 May 1995, analysed on 14 October 2026: 11,475 days. 11,475 = 498 × 23 + 21, so the physical cycle is on day 21 of 23: sin(2π × 21 ÷ 23) = −0.52, i.e. −52%. Emotional cycle on day 23 of 28: −90%. Intellectual cycle on day 24 of 33: −99%. The next zero crossings come in 2 days (physical), 5 days (emotional) and 9 days (intellectual). The three cycles only line up again after 21,252 days, about 58 years.
What the tool does and does not do
The tool shows the three values for a single date; it does not draw a curve or list critical days, which you find by changing the analysis date. The 23, 28 and 33-day cycles match no identified physiological mechanism, unlike the circadian rhythms studied by chronobiology. The results say nothing about health or performance and do not replace medical advice.
Sources and dates: Touitou et al., "The biorhythm concept revisited", Chronobiology International, 2026 (critical review published online 28/03/2026: no experimental validation and no physiological mechanism) (checked 14/10/2026)
Frequently Asked Questions
How do you calculate your biorhythm?
Count the days lived since birth, then compute sin(2π × days ÷ 23) for the physical cycle, ÷ 28 for the emotional one and ÷ 33 for the intellectual one. On day 21 of a 23-day cycle, the physical value is −52%.
What is a critical day in biorhythms?
It is a day when a curve crosses zero, which happens every 11.5 days for the physical cycle, 14 days for the emotional one and 16.5 days for the intellectual one. No link between these days and any increased risk has been established.
Are biorhythms scientifically proven?
No. A critical review published in 2026 in Chronobiology International found no experimental validation and no physiological mechanism for the 23, 28 and 33-day cycles. Recognised biological rhythms, such as the roughly 24-hour circadian rhythm, belong to a different framework.
Why three cycles of 23, 28 and 33 days?
These lengths go back to the work of Wilhelm Fliess in the early 20th century and rest on no physiological measurement. They only line up together once every 21,252 days, about 58 years.
Is this tool really free?
Yes, all tools on Kristools are 100% free and accessible without registration.
Is my data protected?
Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.