Biorhythm Calculator: your physical, emotional and intellectual cycles today

What is biorhythm?

Biorhythm is a theory that three regular cycles run through a person's life from the moment of birth, each rising and falling like a wave:

• Physical cycle, 23 days. Stamina, strength, coordination, resistance to illness. High phase: energy and endurance. Low phase: recovery and fatigue.
• Emotional cycle, 28 days. Mood, sensitivity, creativity, how much you feel what you feel. High: optimism, warmth, expressiveness. Low: irritability, withdrawal, moodiness.
• Intellectual cycle, 33 days. Memory, reasoning, alertness, decision-making. High: sharp thinking, quick learning. Low: fuzziness, second-guessing.

Each cycle is modelled as a sine wave: it starts at zero on your birth date, rises to +100% at a quarter of the cycle, returns to zero at the halfway point, falls to −100% at three quarters, and comes back to zero to begin again. Because the cycle lengths differ, the three waves drift in and out of phase with each other, which is why some days feel like everything is working and others feel like nothing is. All three cycles realign at zero only every 21,252 days, about 58.2 years: the least common multiple of 23, 28 and 33.

Where the theory came from

The 23- and 28-day cycles were proposed around 1900 by Wilhelm Fliess, a Berlin physician and correspondent of Sigmund Freud, and independently by Hermann Swoboda, a psychology lecturer in Vienna. The 33-day intellectual cycle was added in the 1920s by Alfred Teltscher, an Austrian engineering teacher who thought he saw it in his students' performance. Biorhythm became a popular craze in the United States in the 1970s, when paperbacks, pocket calculators and even airport vending machines offered to print your chart.

How to calculate biorhythm by hand

You can do this with a calculator in about a minute:

1. Count the days from your birth date to today, including leap days. Call that number t. (Someone born 1 January 1996 has lived 11,226 days as of 26 September 2026.)
2. For each cycle, divide t by the cycle length (23, 28 or 33) and keep only the remainder. That is where you are in the current cycle.
3. Convert to a percentage: sin(2π × remainder ÷ cycle length) × 100.

For t = 11,226: 11,226 ÷ 23 leaves remainder 2, so the physical cycle is at sin(2π × 2/23) ≈ +52% and rising. 11,226 ÷ 28 leaves remainder 26, so emotional is at sin(2π × 26/28) ≈ −43% and climbing back toward zero. 11,226 ÷ 33 leaves remainder 6, so intellectual is at sin(2π × 6/33) ≈ +91%, near its peak. A day with strong physical and intellectual energy but a low emotional tide: a good day for a workout and a spreadsheet, a poor day for a difficult conversation. The calculator above does the arithmetic and draws the curve.

Critical days: the part of biorhythm people actually use

A critical day is any day a cycle crosses zero, switching from high to low or low to high. The tradition holds that you are least stable on those days: not weak exactly, but unpredictable, with the cycle changing direction under you. Physical critical days come every 11.5 days, emotional every 14, intellectual every 16.5.

When two cycles cross zero within a day of each other it is a double critical day; all three together is a triple critical day, which for most people happens only a handful of times in a lifetime after birth. The 1970s literature was full of claims about accidents clustering on critical days. Later controlled studies did not find the effect, but the habit of moving a little more carefully every couple of weeks costs nothing and is, frankly, good advice on any day.

Is there a biorhythm chart for luck?

Not in the classic model. Physical, emotional and intellectual are the only three cycles in the original theory. Later writers added "secondary" cycles: a 38-day intuitive cycle, a 43-day aesthetic cycle, a 53-day spiritual cycle, and various "luck" or "fortune" cycles that have no shared definition. If a site sells you a biorhythm chart for luck, it is using one of those inventions. Some calculators also combine the three classic cycles into a single average and call a high average "lucky"; that is a summary, not a fourth cycle.

If what you want is a number for the day, the astrological tradition has a better-defined answer than biorhythm: lucky numbers derived from your Sun and Moon signs and the numerology of the date. Our lucky numbers today tool does exactly that, and your personal year number sets the theme for the whole year.

Is biorhythm real? An honest note

Biorhythm theory is pseudoscience. The cycles are not observed in the body; they are assumed from the birth date and drawn with a formula. Studies in the 1970s and 1980s that tested biorhythm against sports results, accident records and exam performance found no predictive power beyond chance. Real biological rhythms exist, but they are different things: the roughly 24-hour circadian rhythm that governs sleep and hormones, the menstrual cycle (which averages about 28 days but varies from person to person, and is not the emotional biorhythm), and seasonal patterns. None of them is fixed by your birth date.

So why offer a calculator? Because biorhythm works well as a self-check ritual. Looking at three numbers every morning and asking "how do I actually feel today?" is a small, useful act of attention. People who track it tend to notice their real patterns, sleep, caffeine, cycle phase, workload, faster than people who do not. Treat the curve as a prompt, not a prophecy. That is also how AstraLink treats astrology: a structured language for reflection, never a diagnosis.

Biorhythm and astrology: how AstraLink uses both

In the AstraLink app, your Daily Focus screen shows the three biorhythm percentages alongside the advice of the day, opportunities and watch-outs generated from your transits, and your lucky numbers and colours. The point is not that biorhythm predicts anything; it is that a morning glance at your cycles, the Moon's phase and sign, and what is touching your chart today gives you a quick, honest read on what kind of day to plan for. The cosmic calendar for today gives you the sky part for free; join early access to get the personal part at launch.

If you are curious how your cycles line up with a partner's, the biorhythm compatibility calculator compares two birth dates. For compatibility that is actually grounded in a tradition, the birth chart compatibility calculator compares Sun and Moon signs and previews the synastry the app computes in full. All our free calculators are listed on the tools page.