Doctors determine their patients' ideal weight based on their height by using the body mass index. A person's BMI is his weight in kilograms divided by the square of his height in meters, explains MedicineNet.
The National Institutes of Health now defines normal weight, overweight and obesity according to BMI rather than traditional height/weight charts, notes MedicineNet. Adolphe Quetelet began using this calculation in the 19th century, and Ancel Keys reinvented it and named it body mass index in the 1950s, according to PubMed Central.
A BMI between 18.5 and 24.9 indicates a health weight, states the National Heart, Lung and Blood Institute. A BMI between 25 and 29.9 means a person is overweight, while 30 or above places him in the obese category. There are limits to BMI as a screening tool, such as overestimating body fat in very muscular people or underestimating it in persons who have lost muscle.
Although BMI is calculated using kilograms and meters, most BMI calculators use a formula that converts these units to pounds and inches. BMI calculators are widely available online, and while BMI can screen for weight categories that may lead to health problems, it is not diagnostic of the body fatness or health of an individual, cautions the Centers for Disease Control and Prevention.