Determine if you're in the healthy range by reading across the the chart. You will need to know your current and exact height and weight. Your age is less important though they are considerations for belly fat and muscle density. Determine your healthy weight range on the chart. Next use the BMI calculator to determine what's known as your index. Your BMI index will be calculated by your weight divided by the square of your height, but the calculation is made simple for you by the calculator. A healthy BMI index is considered to fall between 19 and 22. People within this range have been shown to live longer and have less incidence of heart disease.