The popular calculation of someone's body mass index, or BMI, does attempt to tell you, based on your weight, where you fall in the range of people who are the same height. But you should rid yourself of the idea that there is one "ideal" weight for someone of your height. That's because everyone is built differently, and has a different genetic makeup that makes a certain weight perfect for one person but silly for another. Nevertheless, BMI can give you a good sense of where you fall among people of your height, and can help you realize if you're overweight or obese, or nearing being so.
Flaws with BMI
In a 2006 article on health and obesity, featured by Public Broadcasting Service, food expert Michael Rosenbaum explained that ideally, your body weight would be defined based on health. He says that this health metric would define obesity as "the degree of fatness at which your current or future risk of disease is increased." He notes that this ideal definition isn't practical, and that BMI has become a popular tool to assess body fatness, but he points out its flaw: its measures can label individuals who are very muscular as obese, and conversely can label someone who doesn't weigh that much but who has a high, unhealthy percentage of body fat as not obese. However, taken together with a grain of salt and with the advice of a licensed health care professional, BMI can be a useful tool to use to gauge where your weight places you on the spectrum of healthy ranges for people of your height.
Calculating Body Mass Index
Your body mass index can be calculated by dividing your weight by the square of your height, then multiplying by a conversion factor of 703. This is assuming your weight is measured in pounds and your height in inches.
BMI for Children
The CDC says that though calculating BMI for children and adults is done using the same process, the criteria used to interpret the scores for kids are different. Age- and sex-specific percentiles are used for children because the number of body fat changes with age and because the amount of body fat that's usual differs between girls and boys. To assess where a child's body mass index places him among his peers of the same sex, age and height, visit the CDC's page with the full table of scores.
Interpreting BMI for Adults
The CDC notes that the standard "weight status categories" associated with BMI ranges for adults are as follows: those with a BMI of under 18.5 are underweight; those between 18.5 and 24.9 are considered "normal." Those who have a BMI that falls in the range of between 25.0 and 29.9 are considered overweight, and those with a BMI of 30 and above are considered obese.
Caveats and Alternatives
Though BMI is in general a good predictor of body fatness, it should be kept in mind that correlation does vary by sex, race and age. Also, at the same BMI, women usually have more body fat than men. In addition, at the same BMI older people tend to have more body fat than younger adults. Finally, athletes could have high BMIs because of increased muscle mass, rather than increased body fat. The CDC notes that looking at waist circumference and looking at other risk factors associated with obesity, such as high blood pressure or physical inactivity, can also be predictors of likelihood of developing overweight- or obesity-related diseases. Another tool developed by a University of Nevada statistician discussed in the "Los Angeles Times," called the Maximum Weight Limit, provides people with a number calculated using methods similar to BMI that individuals can use as a "stop sign" equivalent to watch their weight. The creator says knowing their Maximum Weight Limit could help people know that intervention and change are immediately needed, as a sort of alarm bell for health.
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