BMI (Body Mass Index) is a gauge of how your body weight relates to your height. You're most likely to have a discussion about your BMI with medical professionals, such as during a doctor's visit, or when working with a personal trainer. While BMI is far from perfect as a body measurement tool, its ease of use makes it widely applicable and, if nothing else, watching your BMI change over time gives you a way of gauging your progress relative to when you started.
Procedure
To calculate your BMI, either cross-reference your height and weight on a printed BMI chart--this may be available at your doctor's office--or input your height and weight into an online BMI calculator. For calculating a child or teen's BMI, you will also need the subject's age and gender. If you're reading a child or teen's BMI off a chart, make sure you have a chart with the appropriate age and gender for that child.
Interpretation
For adults, interpreting your BMI is as simple as locating where your height and weight intersect on a chart, then reading the appropriate BMI number. If your BMI is below 18.5, you're considered underweight. If your BMI is between 18.5 and 24.9 you're normal weight, between 25 and 29.9 is considered overweight, and a BMI of 30 or above is considered obese.
Percentile
BMI percentiles, also known as BMI-for-age percentiles, apply only to children and teens. These offer a ready way of comparing a child's BMI against those of other children of the same age and gender. Children ranking below the fifth percentile for their gender and age are considered underweight, children over the 85th percentile are considered overweight, and those at or above the 95th percentile are labeled as obese. Normal weight is considered as anything between the fifth and 85th percentiles.
Advantages
Advantages of using BMI to assess one's weight include the fact that it costs nothing and requires no special equipment beyond an Internet connection or a printer chart; one can accurately calculate a BMI with little or no special training; and, because calculating a BMI is so easy and inexpensive, there is no barrier to doing so frequently to measure weight loss progress over time.
Disadvantages
Disadvantages of using BMI to assess one's weight include the fact that women will tend to have more body fat than men with the same BMI; extremely muscular individuals may have a high BMI because the extra muscle, not fat, adds more weight to the body; and, for children and teens, the BMI-for-age provides a comparison against other children as opposed to a true objective standard of measurement.
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