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Why is BMI not a direct measure of body fat?

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Close-up of hands using a measuring tape to measure the waist circumference of an obese man wearing a white shirt and beige pants. — obesity The excessive accumulation of body fat can cause serious medical conditions.

Body Mass Index (BMI) is an estimate of total body fat rather than a direct measurement. It is calculated using only weight and height, which limits its ability to account for individual body composition.123

BMI fails to distinguish between the relative proportions of bone, muscle, and fat. Because bone and muscle are denser than fat, individuals with high muscle mass or strong bone density—such as athletes—may be incorrectly classified as overweight or obese despite having low body fat levels.31

Furthermore, BMI does not account for factors that influence body fat distribution and health, including:
* Demographics: Age, gender, and ethnicity, all of which affect typical body fat percentages.13
* Body Shape: Measurements such as waist circumference, waist-to-hip ratio, or waist-to-height ratio, which provide more nuanced data than weight alone.3

Due to these limitations, the American Medical Association has advised healthcare professionals against relying solely on BMI to assess a patient's health.3