Back in the old days, doctors looked at life insurance tables to determine healthy weights for patients. These listed healthy weight ranges for men and women based on their heights. But life insurance companies had their own tables, and they weren't always the same. So in 1998 the National Institutes of Health unveiled the Body Mass Index (BMI) as a way for everyone to figure out healthy weights in the same manner. To calculate someone's BMI, you divide the person's weight in pounds by his height in square inches, then multiply the result by 703. A good BMI is 18.5-24.9, according to the National Institutes of Health, while a BMI of 25-29.9 means you're overweight. Anything 30 or more? Obese. BMI was quickly adopted by most health professionals [source: Zelman].
In 2014, the BMI was under fire because it doesn't factor in your age, gender or muscle mass. There's also no distinction between lean and fat body mass. Basketball superstar Michael Jordan, for example, had a 27-29 BMI in his prime -- meaning he was overweight -- despite sporting a chiseled frame and less-than-30-inch (76-centimeter) waist. Similarly, elderly people on the roly-poly side might have normal BMIs because they've lost muscle mass, and muscle weighs more than fat. Many health professionals now say you should use BMI as just one measure of health and fitness, along with factors such as body fat percentage, waist circumference and level of physical activity [source: Zelman].