What is the Basal Metabolic Index (BMI)?

The most accurate way to traditionally determine if a person is overweight or obese is to directly measure and quantify excess fat tissue. Unfortunately, this is routinely impractical because it is expensive and difficult to assess.


BMI is a calculation used to determine overweight and obesity in adults and children. It is a measurement that estimates body fat by measuring excess weight for height. BMI is calculated using an accurate weight and height measurement. In children, this number is then plotted on a BMI curve specific for the child’s age and sex and a percentile is calculated.

BMI= weight (kg)/height²(m²)

The downside to BMI is that it does not distinguish between body fat and muscle, and thus BMI is not a perfect measure of body fat. Because of this, a very muscular child will weigh a lot relative to his height, and will have an increased BMI, but will not be overweight. 

Therefore, BMI information alone will not tell you if a child is overweight. If your child has a BMI >85%, make an appointment with your child’s doctor to determine if he is overweight or obese. Your pediatrician should look at the BMI trend, not just one particular BMI calculation to determine if your child is overweight. He will also evaluate your child’s musculature to determine if the BMI is misleading due to increased muscularity.

As you will learn on this program, the BMI trend is more important than the actual number. A child may be gaining excess weight (rapid weight gain) and not fall into the overweight category. Or a child might fall in the overweight category but if this is where he has always tracked, this may be his genetically determined weight. So use a healthy dose of common sense when interpreting BMI and see your child’s doctor with any concerns.

Note: BMI calculation requires an accurate height and weight or the BMI percentile is not accurate. If you cannot do this at home, please make an appointment with your child’s physician.