For children and teens, BMI is age- and sex-specific and is often expressed as a percentile. After BMI is calculated for children and teens, it's plotted on the CDC BMI-for-age growth charts (for either girls or boys) to obtain a percentile ranking. This percentile indicates the relative position of the child's BMI number among children of the same sex and age.

For adults:

• Below 18.5 – Underweight
• 18.5-24.9 – Normal weight
• 25-29.9 – Overweight
• 30 and above – Obese

For children:

• Below 5th percentile – Underweight
• 5th to 85th percentile – Healthy weight
• 85th to 95th percentile – Overweight
• 95th percentile or greater – Obese

BMI percentile shows how a person's BMI compares to others of the same age and gender. For adults, it indicates the percentage of the population that has a BMI at or below that value. For children, it's used to interpret the BMI number because BMI is both age- and sex-specific for children and teens.

While BMI is a useful measure for most people, it does have limitations. It may overestimate body fat in athletes and others who have a muscular build, and it may underestimate body fat in older persons and others who have lost muscle. BMI is not a direct measure of body fat and doesn't account for factors like muscle mass, bone density, age, and sex.

For adults, checking BMI once or twice a year is usually sufficient unless advised otherwise by a healthcare provider. For children and teens, BMI should be checked as part of regular check-ups with a pediatrician, typically annually, as it's important to track growth patterns over time.

If your BMI falls outside the healthy range, it's a good idea to consult with a healthcare provider. They can perform a more comprehensive health assessment and provide personalized advice. Remember, BMI is just one indicator of health and should be considered alongside other factors like diet, physical activity level, and overall health status.