BMI by Age

Body Mass Index (BMI) is calculated the same way at any age, but how the result is interpreted can change depending on a person’s stage of life. Growth, muscle mass, and body composition all shift over time.

You can calculate BMI using height and weight with our BMI calculator, then use age-related guidance to understand the result.

BMI for children and teens

For children and teenagers, BMI is not interpreted using adult number ranges. Instead, BMI is compared to others of the same age and biological sex using percentiles.

These percentiles account for normal growth and development during childhood and adolescence. A BMI that is healthy for one age may not be healthy for another.

Learn more on our BMI for teens page.

BMI for adults

For most adults, BMI is interpreted using fixed categories:

These ranges are associated with health risks at a population level. However, BMI does not distinguish between muscle and fat.

BMI and aging

As people age, muscle mass often decreases while body fat may increase, even if weight stays the same. This means BMI can sometimes underestimate health risks in older adults.

In some cases, slightly higher BMI values may not carry the same risk in older adults as they do in younger people, especially when muscle loss is present.

Using BMI wisely at any age

BMI works best as a screening tool, not a diagnosis. At any age, it should be considered alongside other factors such as:

If you are unsure how to interpret a BMI result, a healthcare professional can help put it into context.

To get your BMI number quickly, use our free BMI calculator.