Body mass index, or BMI, is a way of estimating your body fat content, based on your height and weight. Though BMI isn't an exact science, it gives you a good sense of where you stand on the weight spectrum--underweight, healthy weight, overweight or obese. BMI is expressed as a number, and obesity is defined as anything over 30.
Calculation
To calculate BMI, you need to know your height in meters and your weight in kilograms. Say you're 5 feet, 8 inches tall and 170 pounds. That's equal to 1.73 meters and 77.11 kilograms.
Start by taking your height and multiplying it by itself to get the "square" of your height: 1.73 x 1.73 = 2.99. Now divide your weight by the square of your height: 77.11 / 2.99 = 25.8. Your body mass index is 25.8. BMI is usually expressed to one decimal point.
You can also do the calculation with your height in inches and your weight in pounds, but it involves an extra step. Start out with by squaring your height, which is 68 inches: 68 x 68 = 4624. Now divide your weight by the square of your height: 170 / 4626 = 0.03676. Here's where the extra step comes in. Take that number and multiply it by 703, like this: 0.03676 x 703 = 25.8.
Translation
A body mass index of 18.5 to 24.9 is considered healthy. Sometimes the term "normal" is used, but the average American now has a BMI above this range, so "normal" is not necessarily healthy. A person with a BMI lower than 18.5 is considered underweight, while a BMI over 24.9 is overweight. People with a BMI of 30.0 or above are obese.
Children
There's a separate formula for children and teenagers that takes into account their age, sex, their height to the nearest eighth of an inch and their weight to nearest quarter-pound. The calculation is more complicated than the adult formula outlined above. See the link in the Resources for a children's BMI calculator provided by the Centers for Disease Control and Prevention. Children's BMI is expressed as a percentile of the overall child population. Those below the fifth percentile are considered underweight; those in the 85th percentile and higher are overweight; and those in the 95th percentile and up are obese.
Limitations
According to the National Institutes of Health, the BMI calculation can overstate the body fat of athletes and people with a muscular physique. In such cases, someone could be labeled as obese when he is not. The CDC stresses that BMI is a screening device only, not a diagnostic tool. Two people could have the same BMI, but because of differences such as age, sex and genetic factors, one could be relatively healthy while the other is obese. Your doctor can make the best determination of whether you are obese and what steps you can take to lose weight.
Alternatives
Other ways to measure body fat percentage include measuring the thickness of skin folds, weighing someone underwater, or measuring how an electrical signal passes through the body. These tests involve specialized equipment and special training for the people administering them, however, while BMI can be easily determined with a calculator.
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