Biorhythm calculator
Enter your date of birth to see your four biorhythm cycles on a 31-day chart, today's values with a short reading, and your next critical days. No sign-up needed.
Calculate your biorhythm
All you need is your date of birth. The time doesn't matter, because biorhythms count whole days.
How to calculate your biorhythm
Each cycle is a sine wave that starts at zero on the day you're born. A cycle's value on a given day comes from this formula:
biorhythm = sin(2π × t ÷ T) × 100
- t
- the number of days since your date of birth,
- T
- the length of the cycle: 23, 28, 33 or 38 days.
The result runs from −100 to +100. Plus means a charged phase, minus a recovery phase, and a zero crossing marks a critical day. Your birth time doesn't matter, because only whole days count.
A step-by-step example
Take someone born on July 15, 1990. By January 1, 2026 they had lived 12,954 days: 35 years of 365 days, 9 extra days from leap years and 170 days since their last birthday. The remainder when you divide that number by a cycle's length tells you where on its wave each cycle is:
| Cycle | Day of the cycle | Value |
|---|---|---|
| Physical | 5 of 23 | +98 |
| Emotional | 18 of 28 | −78 |
| Intellectual | 18 of 33 | −28 |
| Intuitive | 34 of 38 | −61 |
On that day the physical cycle is just short of its peak, and the other three are in their recovery phase. The calculator at the top of the page does the same for each of the 31 days on the chart.
How to read your biorhythm chart
A line above zero is the charged phase: according to biorhythm theory, that area of your life is running at full power. Below zero is the recovery phase, a time for rest rather than records. Direction matters too: a rising cycle is heading for its peak, a falling one for its next critical day.
A critical day is a day when the line crosses zero. The physical cycle does this every 11.5 days, the emotional every 14, the intellectual every 16.5 and the intuitive every 19. The emotional cycle lasts exactly four weeks, so its critical days always fall on the same weekday you were born on. Tradition pays the most attention to days when two or three cycles cross zero at once.
Where biorhythms come from
In the late 19th century Wilhelm Fliess, a Berlin ear, nose and throat doctor and a friend of Sigmund Freud, described cycles of 23 and 28 days. The Viennese psychologist Hermann Swoboda pointed to similar periods on his own. In the 1920s Alfred Teltscher, a teacher from Innsbruck, added the 33-day intellectual cycle.
Biorhythms hit their peak of popularity in the 1970s, when computers printed out charts and pocket biorhythm calculators showed up in stores. The intuitive cycle (38 days) was added even later, and not every calculator includes it.
Frequently asked questions
What is a biorhythm?
A biorhythm is a theory that from the day you're born you live in a few fixed cycles: physical (23 days), emotional (28 days) and intellectual (33 days), plus, in newer versions, intuitive (38 days). Each cycle is a wave that rises, falls and crosses zero every half period. The positive phase is said to bring more energy in that area of life, the negative phase a time to recover.
How do you calculate a biorhythm?
Count the days since you were born, then for each cycle calculate sin(2π × days ÷ cycle length). The result falls between −1 and 1 and is usually multiplied by 100. The easiest way is to enter your birth date in the calculator on this page: it works out all four cycles for 31 days at once.
What is a critical day in a biorhythm?
It's a day when a cycle crosses zero and switches from its positive to its negative phase or the other way around. Biorhythm theory treats it as an unstable day, when mistakes or a dip in form come more easily, so it's worth slowing down. The most demanding are said to be days when two or three cycles cross zero at once.
Are biorhythms accurate?
Not as a forecast: studies that tested whether biorhythms predict accidents, sports results or mood found no such link. You can still use your chart as a calendar for watching yourself. If you want to test it on yourself, write down how you feel each evening for a month and compare your notes with the chart.
What does a negative biorhythm mean?
A value below zero marks a recovery phase, not something bad. In biorhythm theory it's a time when that area of life rests: better to pace yourself and not aim for personal records. Tradition pays more attention to critical days, the zero crossings.
Do I need my birth time to calculate a biorhythm?
No. Biorhythms count whole days from your date of birth, so the time and place you were born don't change the result. Astrology is different: a birth chart and a rising sign do need your birth time.
Your biorhythm every day in the Sanctuary
Save your birth date in your profile, and your chart will be waiting for you every day with nothing to type. The Sanctuary also has AI readings: tarot, numerology, your birth chart. You get 5 free tokens to start.
Create a free account