Body Mass Index (BMI) is a way to measure children and adult's risk of obesity. BMI calculations use height and weight to come up with a number that can be used to determine if a person is underweight, healthy, overweight or obese. A score of less than 18.5 indicates that a person is underweight. Scores between 18.5 and 24.9 are normal. A score of 25 and above indicates that a person is overweight, and within that a BMI score of 30 or more indicates obesity. See the Resources section below for a link to an online BMI calculator.
BMI, however, has disadvantages and limitations because it does not account for weight that comes from muscle instead of fat. Athletes and muscular people will not get accurate BMI scores, and neither will pregnant women. As an individual measurement, BMI is not as helpful as body fat percentage tests.
BMI and Height
BMI uses height in relation to weight. It cannot take into account the varying length of people's legs, which will make people of the same height but with different size torsos. A measurement based on height will also be affected by age and gender.
BMI and Special Populations
An article in The International Journal of Obesity and Related Metabolic Disorders said that "Our results indicate that a single BMI standard should not be used: rather, a standard should be developed for each population. This conclusion is in agreement with work by other researchers, who also found BMI inconsistencies between groups." For example, Caucasian children and African-American children with equal BMI scores have different fat measurements. This is because BMI cannot differentiate if weight is coming from fat or muscle.
Alternative
An alternative measurement for obesity that accurately will tell people if they are in a healthy range of fat is to measure body fat percentage. Body fat percentage tests measure a person's fat versus their lean body mass, which includes bones, blood, muscle, and everything other than fat. Figuring out exactly how much fat a person has is a more useful measurement when developing a plan for a healthy lifestyle.
Body fat can be calculated with skinfold tests, which use tools called calipers--little pliers that are used to measure the size of fat on certain sites on the body to overall calculate the body fat percentage. Another way is to use a handheld machine or a scale with body fat percentage panels that will send small amounts of electricity through a person and, judging by the time it takes to get through the body, will determine the body fat percentage.
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