Body mass index (BMI) is a measure of body fat calculated from height and weight measurements. BMI is often used in adults to assess the risk of disease. You may see BMI-for-age growth charts and wonder when they can be used for your child. For children under 2 years of age, weight-for-length growth charts are recommended to be used as opposed to BMI. For children 2 years and older, BMI-for-age growth charts are recommended to monitor growth and identify malnutrition (6).