Established by the federal government as a standard to determine obesity, Body Mass Index is calculated by dividing your weight in kilograms by your height in meters squared. A Body Mass Index between 18.5 and 25 is ideal. People with a BMI between 25 and 29.9 are overweight and a BMI greater than 30 indicates obesity. Being overweight increases the chances of serious health risks like heart disease, diabetes, and high blood pressure. But Body Mass Index is not a foolproof measurement. Though muscle tissue weighs more than fat tissue, BMI calculation doesn't recognize the difference between muscle and fat ­ an aggravating footnote for people who spend a lot of time at the gym. For those who lead more sedentary lives however, BMI can be a useful first indicator of general fitness.