The arithmetic
How to calculate a biorhythm
Count the days since you were born, divide by the length of each cycle, and the remainder tells you where you are. That is the whole method. Here it is step by step, with a worked example and the formula for a spreadsheet.
A worked example
Someone born on 2 November 1988, reading 1 January 2026.
01
Count the days
Count the whole days from the date of birth to the date you want to read. By hand, the quickest way is 365 for every full year, one more for every 29 February in between, then the days since the last birthday.
| 37 full years × 365 | 13,505 |
|---|---|
| 29 Februarys in between | 9 |
| 2 November 2025 to 1 January 2026 | 60 |
| Days since birth | 13,574 |
02
Divide by each cycle
Divide the count by 23, 28 and 33. Each remainder, plus one, is the day of that cycle. The plus one is there because the day you were born is day 1 of every cycle, not day 0.
Physical13,574 ÷ 23 = 590, remainder 4
day 5
Emotional13,574 ÷ 28 = 484, remainder 22
day 23
Intellectual13,574 ÷ 33 = 411, remainder 11
day 12
03
Read the state
Look each day up in its cycle's table to find the state and its symbol. The tables are on How to read a biorhythm chart, and every term is in the glossary.
The three symbols together are the day's combination, one of 512. BioCalendar has a written reading for each, and shows you yours.
- +
ChargingPhysical, day 5 of 23
- *
RechargingEmotional, day 23 of 28
- #
Charged, easingIntellectual, day 12 of 33
The combination+*#
Excel and Google Sheets
In a spreadsheet
Put the date of birth in A2 and the date to read in B2. A spreadsheet stores a date as a day number, so subtracting one from the other gives the count, and MOD gives the remainder. These formulas work in Excel and Google Sheets.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Born | Date | Physical | Emotional | Intellectual |
| 2 | 2 Nov 1988 | 1 Jan 2026 | 5 | 23 | 12 |
- Physical, C2
=MOD(B2-A2,23)+1 - Emotional, D2
=MOD(B2-A2,28)+1 - Intellectual, E2
=MOD(B2-A2,33)+1
Elsewhere
The smooth wave other charts draw
Many biorhythm charts draw each cycle as a smooth wave and give the day a percentage: the sine of 2π times the count, divided by the length of the cycle. On the day of birth all three waves are at zero and rising.
BioCalendar does not draw waves or print percentages. It counts whole days and names one of eight states, because a percentage would suggest a measurement the theory never made.
Check your working
- 01
On the day you were born, all three cycles read day 1.
- 02
The emotional cycle is exactly four weeks long, so 28 days later it is on the same day again, and on the same weekday.
- 03
All three are on day 1 together again after 21,252 days, about 58 years.
The record
What the number means
The arithmetic is exact. The theory it serves was tested and not supported, so read the result the way BioCalendar presents it: as a counted pattern and a prompt to notice the day, not a forecast. The research is on Are biorhythms real?.
Continue the story
Keep going
- 01What is a biorhythm?A biorhythm is a cycle said to begin at birth and repeat on a fixed period: 23 days physical, 28 emotional, 33 intellectual. The theory and the evidence.
- 02How to read a biorhythm chartThe three cycles, the eight states each passes through, the symbols BioCalendar uses for them, and what a critical day, a high and a low mean on the chart.
- 03Biorhythm glossaryThe words and symbols on a biorhythm chart, defined: the three cycles, the eight states and their days, critical days, the midline and the BioRating.