biorhythm.com
How to calculate biorhythm – the formula
There is surprisingly little mathematics behind biorhythm: the number of days lived and a sine function.
1. Count the days lived
First, count the days from the birthday to the date you want. The birthday itself is day 0. Leap years are of course included. Someone born on 1 June 1990 has lived exactly t = 13,285 days on 15 October 2026.
2. The formula
For each cycle with period T (23, 28 or 33 days):
value = sin(2π · t / T)
The result lies between −1 and +1 and is usually given as a percentage. Since the sine repeats after each period, the remainder of t divided by T is enough.
3. Worked example
| Cycle | T | t mod T | sin(2π · remainder / T) |
|---|---|---|---|
| Physical | 23 | 14 | −63 % |
| Emotional | 28 | 13 | +22 % |
| Intellectual | 33 | 19 | −46 % |
4. Critical days
A curve crosses the zero line when t is a multiple of half the period. For the physical cycle (11.5 days) this point often lies between two days. biorhythm.com counts a day as critical when the zero crossing falls within that day, i.e. at most half a day away from the day’s value. More in the article on critical days.
5. The master cycle: 58 years
When are all three curves exactly where they were on the day of birth? Only after the least common multiple of the three periods: lcm(23, 28, 33) = 21,252 days, a little over 58 years and 2 months. Since 23, 28 and 33 are pairwise coprime, that is simply their product.
6. Partner compatibility
For the compatibility of two people only the distance d between their birthdays in days matters. For each cycle:
match = (1 + cos(2π · d / T)) / 2
With the same date of birth the result is 100 %; at exactly half a period apart it is 0 % – the curves then run in exactly opposite directions.
A question of convention
Not every program counts the same way: some start the birthday as day 1 or plot the value at the start of the day instead of the middle. The curves then shift by one day. biorhythm.com follows today’s common convention with the birthday as day 0.