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Biorhythms are natural cycles of energy which continually flow through your emotions, physical body, intellect, and intuition. There have been three cycles in the past that were considered to rule our behavior, the physical, emotional and intellectual. Scientists now believe there is also fourth - intuitional. This cycle reflects intuition, self awareness and spiritual aspects. These body cycles are know as biorhythms.
There are a couple of biorhythm calculators online - these are the best ones I have found:

FREE BIORHYTHM CHARTS

IFate.com
Not only your biorhythms, but there is a new option to chart two people. A good tool for accessing you and your partner at the same time!

Bio-Chart.com
Bio-Chart offers a different style of output than most of the calculators I have found. A little hard on the eyes but a good one for comparison.

MyDailyPrediction.com
This Biorhythm chart provides you with a graphic representation of your high and low energy levels throughout a specific time period. It gives a comprehensive interpretation also, some charts leave you to do your own interpreting, this one does it all for you. Take a look.

FreeBiorhythm.net
Simply enter your birth date and press go!

Procato.com
Another different output - has all the classic cycles plus 7 others. I like the way this one is displayed. Enter your details on the right hand side (it's not so obvious when you first enter the site).

Biorhythmslive.com
Choose from a personal chart, birthday chart and a comparison chart. No interpretations, you do them yourself!