Biorhythm Calculator — 31-Day Chart

What it is

Biorhythm theory claims that three fixed cycles start the day you are born and repeat for life: a physical cycle of 23 days, an emotional cycle of 28 days and an intellectual cycle of 33 days. The model is commonly traced to the ideas of Wilhelm Fliess and Hermann Swoboda, and it had a long run of popularity in magazines and pocket calculators.

To be clear up front, this is not a scientifically proven way to predict anything. It is a fun tool, and it does not forecast or diagnose your health. With that said, it is a pleasant way to play with dates: you can compare your curves with a friend’s, or look ahead to see when all three line up.

The calculator shows today’s value for each cycle as a percentage, whether it is rising or falling, whether today is a critical day and when the next one comes. A chart covers the week before and the three weeks after your reference date, and the same numbers are available in a table. The three lines differ by dash pattern as well as colour, so the chart stays readable if you have trouble telling colours apart. The page starts with an example birth date of 1995-05-15 and today as the reference date. Nothing leaves your browser.

How to use

1. Pick your date of birth. The starting value is only an example.
2. Choose a reference date. It begins at today, and the Today button brings it back.
3. Read the three cards: percentage, rising or falling, and any critical-day mark.
4. Scan the 31-day chart for the trend, or open the table to see each day’s numbers.

How it works

• Your birth date is day 0, and t is the number of days from it to the reference date.
• Each cycle value is sin(2π × t ÷ length), with lengths of 23 (physical), 28 (emotional) and 33 (intellectual) days.
• Values are shown from −100% to 100%, rounded to whole numbers.
• If the next day’s value is higher, the cycle is rising; otherwise it is falling.
• A critical day is a day whose value is exactly 0, or whose sign differs from the next day’s. Every cycle has two per period.
• Both dates must fall between 1900 and 2100, and the birth date cannot be after the reference date.

Examples

Born 1995-05-15, read on 2026-10-04, which is day 11,465.

Cycle	Length	Value	Trend	Critical today	Next critical day
Physical	23 days	14%	Falling	Yes	2026-10-16
Emotional	28 days	22%	Falling	No	2026-10-05
Intellectual	33 days	46%	Falling	No	2026-10-06

A friendly reminder

A low value or a critical day does not mean you will feel worse, perform worse or have an accident. If you feel unwell or run down for a while, see a doctor rather than a chart. Curious about exact ages or day counts? Try the age calculator or the date difference calculator.