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Why is BMI interpreted differently for children than for adults?

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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.

BMI is interpreted differently for children than for adults because children are still in a state of physical growth. While adult BMI categories are fixed, BMI levels for individuals under the age of 20 must be compared with those of other people of the same age and sex to account for these developmental changes.12

For adults over age 20, BMI is calculated using a standard formula—weight in kilograms divided by the square of height in meters—and is categorized into universal weight status designations, such as healthy, overweight, or obese. However, because children's bodies change significantly as they mature, a static number cannot be applied in the same way.21

It is important to note that BMI is an estimate of total body fat rather than a direct measurement, and it does not account for individual variations in bone density, muscle mass, or body composition. Consequently, health professionals are increasingly encouraged not to rely solely on BMI when assessing a patient's health.213