BMI stands for Body Mass Index; it's a way to estimate whether or not an adult is over or under weight for his or her height. No equipment is necessary. You can figure out your BMI at home with a calculator. You multiply your weight in pounds by the number, 705. Then you divide that number by your height in inches squared. The final number is your Body Mass Index.
A healthy BMI is considered to be anything between eighteen and just under twenty-five. If your number is twenty-five or higher the BMI estimates that you're overweight. Over thirty, and the BMI says you're obese. Keep in mind though that these standards have met criticism for being too conservative.
Though the BMI can be a good method for estimating whether one is overweight, it is not at all perfect. The BMI is really just a height to weight ratio chart that is based on thousands of observations about the relationship between height and weight.
The most common criticism of the BMI is that it cannot tell the difference between fat and muscle. Because muscle is denser than fat an athlete's BMI might indicate that he is overweight, but that extra weight might be composed more of muscle than fat. But it's excess fat that typically puts people at health risks. There is no perfect method for figuring out your body fat, but the BMI gives a good estimate.