Q&A: What Does My Body Mass Index (BMI) Mean?

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Question: I’ve heard that my BMI can help me determine whether my weight is healthy, but I’m not sure what a BMI is or what it means.

Answer: BMI is short for body mass index, which is one of several methods for assessing body composition. Body composition is the amount of fat on the body, relative to the amount of lean tissue (everything other than fat). Simply stated, BMI is a ratio of weight to height, calculated as*:

BMI = [(weight in pounds)/(height in inches)²] x 703
BMI = (weight in kilograms)/(height in meters)²

For those who would rather not do the math, the National Heart Lung and Blood Institute has a BMI calculator available online.

BMI categories are*:

Less than 18.5 = Underweight
18.5-24.9 = Normal Weight
25-29.9 = Overweight
30+ = Obese

*Note that BMI is calculated using these same formula for children and teens, it’s interpreted differently.

An individual’s BMI is useful to clinicians, as it can help predict whether the patient is at higher risk for certain diseases. For instance, those with BMI values that place them in the overweight or obese categories are at increased risk of high blood pressure, heart attack, stroke, and type 2 diabetes. Those with BMI values that place them in the underweight category are at increased risk of osteoporosis, particularly in the case of women.

While BMI is a useful tool, both to healthcare practitioners and to individuals who want to track their body status over time, it’s not equally accurate in all populations. While BMI is relatively reliable for individuals of average musculature, very muscular individuals will often have BMI values that overestimate their body fat, as these individuals are generally heavier than those with less muscle. Similarly, those with excessively limited muscle (who are, perhaps, particularly sedentary or who have a wasting disease) may not get an accurate estimate of body fat through BMI; their BMI values will tend to underestimate their body fat status. These systematic failures of BMI to accurately assess body fat in certain populations are due to the fact that muscle is significantly heavier than fat. Still, notes the U.S. Centers for Disease Control and Prevention, the correlation between body fat and BMI is strong for most individuals [1].

While it’s certainly possible to calculate and keep track of your own BMI, it’s best to discuss your body fat status with a physician before making decisions about gaining or losing weight on the basis of BMI value alone.

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