Biorhythm calculator
A biorhythm chart models three sine waves that start on the day you were born: physical (23 days), emotional (28 days) and intellectual (33 days). Enter your birth date to see where each one is today. Nothing you type leaves this page.
Your cycles today
Enter your birth date. The page uses the same formulas as the Heptyx app, and nothing you enter is sent or saved.
The three cycles
Each cycle is a sine wave that starts at zero on the day you were born and repeats for life.
| Cycle | Length | Formula |
|---|---|---|
| Physical | 23 days | sin(2π · t ÷ 23) |
| Emotional | 28 days | sin(2π · t ÷ 28) |
| Intellectual | 33 days | sin(2π · t ÷ 33) |
t is the number of calendar days since your birth day. Heptyx shows each value as a percentage from −100% to +100%, and turns the average of the three into a rhythm score from 0 to 100: 50 + 50 × average.
A worked example
Someone born on 15 May 1990 has lived 13,301 days on 14 October 2026.
| Cycle | Value | Direction |
|---|---|---|
| Physical | +94.2% | falling |
| Emotional | +22.3% | rising |
| Intellectual | +37.2% | rising |
The rhythm score is 76. In the Heptyx app, a 30-second chakra check-in is added at 40%: with answers averaging 3.43 out of 5, the Energy Score that morning is 70.
Critical days, peaks and lows
A critical day is the day a cycle crosses zero. Because 28 is even, the emotional cycle sits exactly on zero every 14 days; the other two cross between two days. A peak is the highest day of a cycle, a low its lowest.
For the example above, from 14 October 2026 over the next month:
- Mon, Oct 19Critical: physical
- Tue, Oct 20Peak: emotional
- Tue, Oct 20Peak: intellectual
- Sat, Oct 24Low: physical
- Tue, Oct 27Critical: emotional
- Thu, Oct 29Critical: intellectual
- Fri, Oct 30Critical: physical
- Tue, Nov 3Low: emotional
- Thu, Nov 5Peak: physical
- Fri, Nov 6Low: intellectual
- Tue, Nov 10Critical: emotional
- Wed, Nov 11Critical: physical
Biorhythm compatibility
Two people’s cycles of the same length keep a fixed gap of d days between their birth dates. Heptyx scores each cycle as 50 × (1 + cos(2π · d ÷ P)): 100 when the waves move together, 0 when they move in opposite directions. There is no single standard formula for this; it is our product choice, and it is shown in the app.
For birth dates 15 May 1990 and 3 August 1992 (d = 811): physical 47, emotional 99, intellectual 6, overall 50. The match never changes; what changes day to day is whether both rhythms are high at once, which Heptyx calls a sync day.
What the evidence says
Biorhythm theory dates from the late 19th century. A 1998 review of 134 studies found no evidence that the cycles predict anything. Heptyx keeps the cycles because many people enjoy them as a daily rhythm to reflect on, and it says plainly that they are a tradition, not a forecast.
Questions
Does my birth time matter?
No. Biorhythm counts whole calendar days from your birth day, which is day 0. Heptyx uses the date on your own calendar and never asks for a time or place of birth.
Why does another calculator show a different value?
Some calculators count your birth day as day 1, use a different time zone, or round differently. Heptyx counts the birth day as day 0 and shows each step, so you can check the arithmetic.
Is biorhythm scientific?
No. A 1998 review of 134 studies found no evidence that biorhythms predict anything. Heptyx offers the cycles as a daily ritual for reflection and says so wherever a number appears.
What is a critical day?
A day on which a cycle crosses the zero line, moving from its positive half to its negative half or back. Heptyx calls it a wavy day and suggests a calm pace, nothing more.
Sources
- en.wikipedia.orgAccessed 14 October 2026
- rosettacode.orgAccessed 14 October 2026
- gnu.orgAccessed 14 October 2026
- skepdic.comAccessed 14 October 2026