Obesity Unit: Calculating BMI
What is BMI?
Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI is a fairly
reliable indicator of body fatness for most people. BMI does not measure body fat directly, but
research has shown that BMI correlates to direct measures of body fat. BMI can be considered an
alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform
method of screening for weight categories that may lead to health problems.
For children and teens, BMI is age- and sex-specific and is often referred to as BMI-for-age.
How is BMI used?
BMI is used as a screening tool to identify possible weight problems for adults and children. However,
BMI is not a diagnostic tool. For example, a child may have a high BMI for age and sex, but to
determine if excess weight is a health risk, a health care provider would need to perform further
assessments. These assessments might include skinfold thickness measurements, evaluations of diet,
physical activity, family history, and other appropriate health screenings.
For children, BMI is used to screen for obesity, overweight, healthy weight, or underweight. CDC and
the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight and
obesity in children beginning at 2 years old.
How is BMI calculated and interpreted?
Calculating and interpreting BMI involves the following steps:
1. Before calculating BMI, obtain accurate height and weight measurements.
2. Calculate the BMI and percentile using the formula:
(weight in pounds)______
BMI = (height in inches) (height in inches)
BMI Categories for adults age 20 and over:
Underweight = <18.5
Normal weight = 18.5-24.9
= BMI of 30 or greater