HEALTH & BODY COMPOSITION

BMI by Age: What the Number Means

Body Mass Index (BMI) is a screening measure based on height and weight. For adults, standard BMI categories are generally applied without separate adult cutoffs for each age. Age is still important when considering the wider picture of health, fitness and body composition.

What BMI measures

BMI uses height and weight to produce a number that can help describe weight relative to height. It does not directly measure body fat, muscle mass, bone density or where fat is stored.

Adult BMI categories

For adults, commonly used screening categories are: underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obesity at 30.0 or higher. These categories are screening ranges rather than a complete diagnosis of an individual's health.

Why age still matters

Adults can experience changes in muscle mass, body composition, activity and health as they age. Two adults with the same BMI may therefore have different body-composition profiles and different health considerations.

Why BMI is not enough by itself

A person with substantial muscle mass may have a higher BMI without having high body fat. BMI can also miss differences in fat distribution. For a broader view, consider other measurements and personal health factors.

BMI and body fat are different

BMI is calculated from height and weight, while body-fat estimates attempt to describe the proportion of body weight that is fat. They answer different questions and should not be treated as interchangeable measurements.

How to use a BMI result

Use the result as a screening starting point rather than a diagnosis. Look at trends over time and consider activity, nutrition, waist measurements, medical history and other relevant information. For children and teenagers, adult BMI categories should not be used; age- and sex-specific growth references are used instead.

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