Free Biorhythm Calculator

Biorhythm theory says that three cycles start on the day you are born and keep repeating at fixed lengths for the rest of your life: a 23-day physical cycle, a 28-day emotional cycle and a 33-day intellectual cycle. Each one swings between a high and a low, and a biorhythm chart plots all three on the same graph so you can see where you stand on a given day.

The idea goes back to around 1900. Wilhelm Fliess, a Berlin physician, proposed the 23-day and 28-day rhythms, and his friend Hermann Swoboda, a psychologist in Vienna, developed them independently. The 33-day intellectual cycle was added later. Biorhythms had their biggest moment in the 1970s, when books, wall charts and pocket gadgets let anyone plot their own curves.

This page is a free biorhythm calculator built on that classic model. You can check biorhythm today or pick any other date, past or future.

The math is one sine wave per cycle. Count the days from your birth date to the day you want to check, divide by the cycle length (23, 28 or 33), multiply by 2π and take the sine. In short: sin(2π × days since birth / period).

The result runs from −1 to +1, which the calculator shows as −100% to +100%. At birth all three curves sit at zero and start climbing. The physical cycle reaches its first peak around day 6, the emotional around day 7 and the intellectual around day 8, then each falls back through zero at its halfway point and bottoms out before rising again.

Because 23, 28 and 33 share no common factor, the three curves only line up at their starting position again after 21,252 days, a little over 58 years.

The physical cycle (23 days) is traditionally tied to energy, stamina, strength and coordination. When the curve is high, the theory says you feel capable and rested. When it is low, you may want to take things slower.

The emotional cycle (28 days) is linked to mood, sensitivity, creativity and how you relate to other people. A high point is read as optimism and openness, a low point as being more easily affected by small things.

The intellectual cycle (33 days) is linked to memory, focus, logic and learning. Highs are described as a good time for study and analysis, lows as a stretch where concentration takes more effort.

A critical day is when a curve crosses the zero line, moving from positive to negative or the other way around. Biorhythm writers treat these moments as unstable transitions: the cycle is changing direction, so that part of you may feel less predictable than usual.

Each cycle crosses zero every half period, so the physical curve does it about every 11.5 days, the emotional every 14 and the intellectual every 16.5. The calculator lists every critical day for all three cycles in the next 30 days. A day when two curves cross at once is called a double critical day and is traditionally seen as more noticeable.

• The value of each cycle on your chosen day, from −100% to +100%, and whether it is rising or falling.
• A chart of all three curves for the 15 days before and the 15 days after that day.
• The critical days coming up in the next 30 days.
• A short note for each cycle that fits where it is now: high, low or critical.

It is free and needs no sign-up. Your birth date is only used to draw the chart.

Researchers have tested biorhythm theory several times, looking at sports results, accident records and test scores. The studies have not found that the cycles predict how people perform or whether accidents happen, and the theory is not accepted by mainstream science.

So treat the chart as a prompt for reflection and a bit of fun. If a low day on the curve makes you stop and ask how you actually feel, that can be useful. Rest, sleep and your own judgment are better guides for decisions than a sine wave. This calculator is for entertainment purposes only.