Biorhythm Calculator
Physical, emotional and intellectual cycles, critical days and compatibility
A biorhythm calculator counts the days since your birth and plots three sine waves: a 23-day physical, a 28-day emotional and a 33-day intellectual cycle. Born on April 2, 1988? On October 10, 2026 you are 14,070 days old: physical -99.8%, emotional 0% (a critical day) and intellectual +75.6%. Biorhythm theory is not supported by scientific evidence, so treat the chart as entertainment.
Last updated October 2026
Method: Each cycle is a sine wave that starts at zero on the day of birth: value = sin(2π × t ÷ P), with t the whole days since birth and P = 23, 28 or 33 days. A day is critical when its curve crosses zero during that day. Compatibility scores each cycle as (1 + cos(2π × offset ÷ P)) ÷ 2, where the offset is the gap between two birth dates modulo P.
Included: The three classic cycles, four optional secondary cycles (38, 43, 48 and 53 days), a 31-day chart, critical days with double and triple flags, daily values, the next critical day per cycle and two-person compatibility.
Not included: Time of birth (whole days only), composite cycles and any prediction or advice. Studies have not found that biorhythms predict performance, accidents or illness; this tool is for fun.
〰️ Your biorhythm
〰️ Days since birth
Critical day for: Emotional. Next critical day: Wednesday, October 14, 2026 (Intellectual).
📈 31-day biorhythm chart
The middle line is zero; the top and bottom lines mark +100% and -100%. Circles mark the moments the physical, emotional and intellectual curves cross zero (critical days). The dashed vertical line is the chart date.
Critical days in this window
- Sat, September 26, 2026: Emotional
- Mon, September 28, 2026: Intellectual
- Sun, October 4, 2026: Physical
- Sat, October 10, 2026: Emotional
- Wed, October 14, 2026: Intellectual
- Fri, October 16, 2026: Physical
- Sat, October 24, 2026: Emotional
Show the daily values
| Date | Physi | Emoti | Intel |
|---|---|---|---|
| Fri Sep 25 | +52.0% | -22.3% | -54.1% |
| Sat Sep 26 * | +73.1% | 0.0% | -37.2% |
| Sun Sep 27 | +88.8% | +22.3% | -18.9% |
| Mon Sep 28 * | +97.9% | +43.4% | 0.0% |
| Tue Sep 29 | +99.8% | +62.3% | +18.9% |
| Wed Sep 30 | +94.2% | +78.2% | +37.2% |
| Thu Oct 1 | +81.7% | +90.1% | +54.1% |
| Fri Oct 2 | +63.1% | +97.5% | +69.0% |
| Sat Oct 3 | +39.8% | +100.0% | +81.5% |
| Sun Oct 4 * | +13.6% | +97.5% | +91.0% |
| Mon Oct 5 | -13.6% | +90.1% | +97.2% |
| Tue Oct 6 | -39.8% | +78.2% | +99.9% |
| Wed Oct 7 | -63.1% | +62.3% | +99.0% |
| Thu Oct 8 | -81.7% | +43.4% | +94.5% |
| Fri Oct 9 | -94.2% | +22.3% | +86.6% |
| Sat Oct 10 * | -99.8% | 0.0% | +75.6% |
| Sun Oct 11 | -97.9% | -22.3% | +61.8% |
| Mon Oct 12 | -88.8% | -43.4% | +45.8% |
| Tue Oct 13 | -73.1% | -62.3% | +28.2% |
| Wed Oct 14 * | -52.0% | -78.2% | +9.5% |
| Thu Oct 15 | -27.0% | -90.1% | -9.5% |
| Fri Oct 16 * | 0.0% | -97.5% | -28.2% |
| Sat Oct 17 | +27.0% | -100.0% | -45.8% |
| Sun Oct 18 | +52.0% | -97.5% | -61.8% |
| Mon Oct 19 | +73.1% | -90.1% | -75.6% |
| Tue Oct 20 | +88.8% | -78.2% | -86.6% |
| Wed Oct 21 | +97.9% | -62.3% | -94.5% |
| Thu Oct 22 | +99.8% | -43.4% | -99.0% |
| Fri Oct 23 | +94.2% | -22.3% | -99.9% |
| Sat Oct 24 * | +81.7% | 0.0% | -97.2% |
| Sun Oct 25 | +63.1% | +22.3% | -91.0% |
* critical day for at least one primary cycle.
🔄 Where each cycle stands
| Cycle | Value | Day | Next critical |
|---|---|---|---|
| Physical | -99.8% ↗ | 18 of 23 | Oct 16 (+6) |
| Emotional | 0.0% ↘ | 15 of 28 | Oct 24 (+14) |
| Intellectual | +75.6% ↘ | 13 of 33 | Oct 14 (+4) |
🤝 Biorhythm compatibility
The two birth dates are 902 days apart. Each cycle is offset by that gap modulo its length; 100% means the two curves run in step, 0% means they mirror each other.
Biorhythm calculator: how the chart works and what it can and cannot tell you
A biorhythm chart is a neat piece of applied trigonometry: three sine waves of different lengths, all switched on at birth. This page shows exactly how the curves, critical days and compatibility scores are computed, and what decades of research concluded about the theory.
What a biorhythm chart shows
Biorhythm theory claims that three fixed cycles start at birth and run unchanged for life. The physical cycle (23 days) was said to govern strength and coordination, the emotional cycle (28 days) mood and sensitivity, and the intellectual cycle (33 days) memory and reasoning. The upper half of each wave was called a high phase, the lower half a recharging phase, and the zero crossings unstable critical days.
The 23- and 28-day cycles go back to Wilhelm Fliess and Hermann Swoboda in the early 20th century, which is why papers still speak of the Fliess-Swoboda theory; the 33-day cycle was added later. According to a comprehensive research review, the idea became popular with the general public in the late 1960s.
The biorhythm formula
Let t be the whole days between birth and the chart date, and P the cycle length:
value = sin(2π × t ÷ P) × 100% day in cycle = (t mod P) + 1 compatibility = (1 + cos(2π × (gap mod P) ÷ P)) ÷ 2 On the day of birth every curve is at 0% and rising. It peaks at +100% a quarter of the way through its cycle (5.75 days after the start for the physical cycle), returns to zero halfway, bottoms out at -100% at three quarters and closes the cycle at zero.
Worked example: April 2, 1988 on October 10, 2026
Someone born on Saturday, April 2, 1988 is 14,070 days old on Saturday, October 10, 2026. The three remainders do the work:
- Physical: 14,070 mod 23 = 17, day 18 of 23: sin(2π × 17 ÷ 23) = -99.8%, just past the bottom and rising.
- Emotional: 14,070 mod 28 = 14, exactly half a cycle: sin(π) = 0%, crossing from positive to negative, a critical day.
- Intellectual: 14,070 mod 33 = 12, day 13 of 33: +75.6%, past its peak and falling.
The next critical days are intellectual on Wednesday, October 14, physical on Friday, October 16, and emotional on Saturday, October 24:
| Date (day of life) | Physical | Emotional | Intellectual | Critical |
|---|---|---|---|---|
| Sat, Oct 10 (14,070) | -99.8% | 0.0% | +75.6% | Emotional |
| Sun, Oct 11 (14,071) | -97.9% | -22.3% | +61.8% | - |
| Mon, Oct 12 (14,072) | -88.8% | -43.4% | +45.8% | - |
| Tue, Oct 13 (14,073) | -73.1% | -62.3% | +28.2% | - |
| Wed, Oct 14 (14,074) | -52.0% | -78.2% | +9.5% | Intellectual |
| Thu, Oct 15 (14,075) | -27.0% | -90.1% | -9.5% | - |
| Fri, Oct 16 (14,076) | 0.0% | -97.5% | -28.2% | Physical |
The intellectual curve reads +9.5% on October 14 and -9.5% the next day; the crossing falls halfway through the 14th, so that day carries the flag.
Critical days, double and triple critical days
The 28-day cycle crosses zero exactly every 14 days. The 23- and 33-day cycles cross at half days, so the calculator flags the day containing the crossing and the gaps alternate. In the first 100 days of life:
- Physical: days 0, 11, 23, 34, 46, 57, 69, 80 and 92.
- Emotional: days 0, 14, 28, 42, 56, 70, 84 and 98.
- Intellectual: days 0, 16, 33, 49, 66, 82 and 99.
Critical days are common. In one complete 21,252-day pattern, 4,006 days are critical for one cycle, 312 for two and 8 for all three: 4,326 days, or 20.4%. With one day in five "critical" for something, believers always found matches. The first triple critical day after birth is day 7,210, at about 19.7 years; for the April 2, 1988 example, December 29, 2007.
Worked example: biorhythm compatibility
Both people's curves run at the same speed, so only the gap between the birthdays matters. April 2, 1988 and September 21, 1990 are 902 days apart:
- Physical: 902 mod 23 = 5 days offset: (1 + cos(2π × 5 ÷ 23)) ÷ 2 = 60.2%.
- Emotional: 902 mod 28 = 6 days offset: 61.1%.
- Intellectual: 902 mod 33 = 11, a third of the cycle: exactly 25.0%.
The average is 48.8%. The score is the correlation of the two curves, rescaled from -1 to +1 onto 0% to 100%, and since the gap never changes, neither does the score:
| Days between births | Physical | Emotional | Intellectual | Average |
|---|---|---|---|---|
| 0 (same birthday) | 100.0% | 100.0% | 100.0% | 100.0% |
| 3 | 84.1% | 89.1% | 92.1% | 88.4% |
| 7 | 33.3% | 50.0% | 61.8% | 48.3% |
| 14 | 11.2% | 0.0% | 5.6% | 5.6% |
| 30 | 33.3% | 95.0% | 92.1% | 73.5% |
| 100 | 21.2% | 5.0% | 99.1% | 41.7% |
| 182 | 92.7% | 0.0% | 0.2% | 31.0% |
| 365 | 84.1% | 98.7% | 96.4% | 93.1% |
| 730 | 46.6% | 95.0% | 86.2% | 75.9% |
| 1,000 | 0.5% | 38.9% | 33.6% | 24.3% |
Births two weeks apart give the lowest average in the table (5.6%) because the emotional curves mirror each other. Births 365 days apart score 93.1%: a year is one day more than 13 emotional cycles, two days more than 11 intellectual cycles and three days short of 16 physical cycles.
Where the cycles stand at milestone ages
The curves depend only on the day count, so everyone shares the same values at the same age in days (arrows show rising or falling):
| Days alive | Physical | Emotional | Intellectual |
|---|---|---|---|
| 1,000 | +13.6% ↓ (critical) | -97.5% ↓ | +94.5% ↓ |
| 5,000 | +63.1% ↓ | -43.4% ↓ | -9.5% ↓ |
| 10,000 | -97.9% ↑ | +78.2% ↑ | +18.9% ↑ |
| 15,000 | +88.8% ↑ | -97.5% ↓ | -28.2% ↓ |
| 20,000 | -39.8% ↓ | +97.5% ↓ | +37.2% ↑ |
| 21,252 | 0.0% ↑ (critical) | 0.0% ↑ (critical) | 0.0% ↑ (critical) |
| 25,000 | -27.0% ↑ | -78.2% ↑ | -45.8% ↓ |
| 30,000 | +81.7% ↓ | +43.4% ↓ | +54.1% ↑ |
Day 21,252 is the full reset: the cycle lengths share no common factor, so the curves realign only after 23 × 28 × 33 = 21,252 days, about 58.2 years. For the April 2, 1988 example that is June 9, 2046.
What the research says
The theory makes a testable prediction: accidents, errors and illness should cluster on critical days and in low phases. In a 1998 review in Psychological Reports, T. M. Hines of Pace University examined 134 studies, published and unpublished. The 35 that reported some support had methodological and statistical errors that explained the findings; the other 99 found no support. The review concluded that biorhythm theory is not valid.
- A 1978 study of 205 highway crashes (135 fatal, 70 nonfatal) in which the driver was clearly at fault found no relationship with critical or minus periods.
- A 1978 analysis of 400 mining accidents in two industries found that accidents on critical days, and in rising versus falling phases, did not differ from chance.
- A 2011 study of 11,395 heart attack admissions in Sherbrooke, Quebec, found no relation to single, double or triple critical days.
Biorhythms versus your real body clock
The body does have real rhythms. The National Institute of General Medical Sciences describes circadian rhythms as physical, mental and behavioral changes over a 24-hour cycle, coordinated by a master clock in the brain that controls melatonin according to how much light the eyes receive. Real rhythms respond to light, sleep, travel and illness; biorhythm theory assumed perfectly regular waves fixed at birth that nothing could shift. Jet lag is a real rhythm problem; a "physical low" on a biorhythm chart is not.
How to use it, and who it is for
- Enter a birth date and, optionally, a chart date; the chart covers 15 days on either side.
- Read the critical days under the chart, or open the daily table for exact values.
- Tick the secondary cycles to add the 38-, 43-, 48- and 53-day curves.
- Enter a second birth date to compare two people's cycles.
It suits anyone curious about the biorhythm craze, party fun, and teachers who want a friendly example of sine waves, remainders and least common multiples. It is not meant for decisions about driving, training, health or relationships.
Key terms
- Period: the days a curve takes to return to its start: 23, 28 or 33.
- Critical day: a day on which a curve crosses zero; double and triple critical days involve two or three cycles.
- Offset: the gap between two birth dates modulo the cycle length.
- Least common multiple: 21,252 days, after which all three classic cycles realign.
What changes the result
Only the day count and the conventions applied to it. Counting the birth day as day 1 instead of day 0 shifts every curve by a day, and so can a birth near midnight in another time zone. Evaluating at noon moves each value by half a day, and some programs flag a critical day only when a value is within a few percent of zero. When counting by hand, a forgotten February 29 shifts all three curves. This calculator counts whole calendar days with the birth day as day 0.
Tips and limitations
- Roughly half of all days are in a low phase for any given cycle, so a bad day in a low phase is no surprise.
- When comparing with another site, check whether it counts from day 0 or day 1.
- The secondary cycles carry no more evidence than the main three.
- The calculator works in whole days and predicts nothing about performance, health, safety or relationships.
Related calculators
- For the weekday you were born and the days to your next birthday, use the Birthday Calculator; for days between dates, the Date Calculator.
- To explore the sine function, use the Trigonometry Calculator; to check the 21,252-day reset, enter 23, 28 and 33 in the LCM Calculator.
- For questions about chance and coincidence, use the Probability Calculator.
Sources
The curves are deterministic trigonometry. The evidence statements rest on these primary sources:
- Hines, T. M. "Comprehensive review of biorhythm theory." Psychological Reports 83(1):19-64, 1998. PubMed 9775660.
- Shaffer, J. W. et al. "Biorhythms and highway crashes. Are they related?" Archives of General Psychiatry 35(1):41-46, 1978. PubMed 619838.
- Persinger, M. A. et al. "No evidence for relationship between biorhythms and industrial accidents." Perceptual and Motor Skills 46(2):423-426, 1978. PubMed 662540.
- Joncas, S. X. et al. "Biorhythm theory does not predict admission for acute myocardial infarction." Journal of Alternative and Complementary Medicine 17(2):143-146, 2011. PubMed 21299417.
- National Institute of General Medical Sciences (NIH), Circadian Rhythms fact sheet.
⚠️ Common mistakes & edge cases
Starting the count at day 1
The birth day is day 0. Starting at 1 moves every curve a day forward and every critical day a day early.
Treating a low phase as a warning
Studies of crashes, mining accidents and heart attacks found no clustering on critical days. Never decide whether to drive or see a doctor based on a biorhythm chart.
Reading 0% as the worst value
In the theory, 0% is a transition. The lowest point is -100%, three quarters of the way through the cycle, such as day 18 of the physical cycle.
Skipping leap days when counting by hand
Over 38 years a person passes nine or ten February 29ths; leaving them out shifts every curve.
Comparing charts built on different rules
Day 0 versus day 1, evaluation time and the critical-day rule differ between sites; charts a day apart can both be right by their own convention.
Reading compatibility as relationship advice
The score depends only on the days between two birthdays. A 5.6% score for siblings born two weeks apart says nothing about how they get along.
❓ Frequently asked questions
What is a biorhythm calculator?
A tool that counts the days since your birth and turns them into three sine waves: a 23-day physical, a 28-day emotional and a 33-day intellectual cycle, each swinging between +100% and -100%. It charts the 31 days around a chosen date, marks critical days and compares two birth dates. The theory behind it is not supported by scientific evidence.
How do you calculate biorhythms?
Count the whole days t from birth to the chart date and compute sin(2 x pi x t / P) for each cycle length P. For someone born on April 2, 1988, October 10, 2026 is day 14,070: 14,070 mod 23 = 17 gives a physical value of -99.8%, 14,070 mod 28 = 14 gives exactly 0 emotional, and 14,070 mod 33 = 12 gives +75.6% intellectual.
What is a critical day in a biorhythm chart?
A day on which a cycle crosses the zero line. Each cycle has two per period. The emotional cycle crosses exactly every 14 days; the physical and intellectual cycles cross at half days, so their critical days alternate between gaps of 11 and 12 days and of 16 and 17 days. Studies found no link between critical days and accidents or illness.
How does biorhythm compatibility work?
Take the number of days between the two birth dates modulo each cycle length. This calculator scores each cycle as (1 + cos(2 x pi x offset / P)) / 2: 100% means the curves run in step, 0% means they mirror each other. Birth dates 902 days apart, such as April 2, 1988 and September 21, 1990, score 60.2%, 61.1% and 25.0%, an average of 48.8%.
Are biorhythms scientifically proven?
No. A 1998 review in Psychological Reports examined 134 studies. The 35 that claimed some support contained methodological and statistical errors, the other 99 found no support, and the review concluded that the theory is not valid. A later study of 11,395 heart attack admissions also found no relation to critical days.
What do the physical, emotional and intellectual cycles mean?
In the theory, the 23-day physical cycle stood for strength, stamina and coordination, the 28-day emotional cycle for mood and sensitivity, and the 33-day intellectual cycle for memory and reasoning, with high phases above zero and recharging phases below. These are claims of the theory, not established facts.
What are the secondary biorhythm cycles?
Some programs add an intuitive cycle of 38 days, an aesthetic cycle of 43 days, an awareness cycle of 48 days and a spiritual cycle of 53 days. They came long after the original three and have no more scientific support. Tick the option to draw them as dashed lines.
Why do biorhythm websites show different values for the same birth date?
Mostly conventions: some count the birth day as day 1 instead of day 0, which shifts every curve by a day; others evaluate the curves at noon, round differently or use their own rule for a crossing in the middle of a day.
Do biorhythm cycles ever repeat?
Yes. Because 23, 28 and 33 share no common factor, all three return to their starting point together only every 23 x 28 x 33 = 21,252 days, about 58.2 years. For someone born on April 2, 1988 that day is June 9, 2046.
Is a biorhythm the same as a circadian rhythm?
No. Circadian rhythms are real: the National Institute of General Medical Sciences describes them as physical, mental and behavioral changes over a 24-hour cycle, kept in step by a brain clock that responds to light. Biorhythms are fixed waves supposedly set at birth, an idea the evidence does not support.
💡 Good to know
One day in five is a critical day
Across a full 21,252-day pattern, 20.4% of days are critical for at least one cycle, so a bad day on a critical day is a coincidence waiting to happen.
Compatibility is fixed for life
Two people's curves move at the same speed, so their offset never changes. Today's score is the score on any other date.
A built-in math lesson
The chart combines the sine function, remainders (t mod P) and the least common multiple. Shift the birth date by 23 days and only the physical curve stays in place.