American Obesity Treatment Association - Treating Obesity one person at time.

Body Mass Index (BMI) is a number calculated from a person's weight and height. BMI is a measurement used to indicate obesity and morbid obesity in adults. 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, such as underwater weighing and dual energy x-ray absorptiometry (DXA).1, 2 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.

How is BMI used?

BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI. However, to determine if excess weight is a health risk, a healthcare 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.

Why does AOTA use BMI to measure overweight and obesity?

Calculating BMI is one of the best methods for population assessment of overweight and obesity. Because calculation requires only height and weight, it is inexpensive and easy to use for clinicians and for the general public. The use of BMI allows people to compare their own weight status to that of the general population.

Waist circumference is a less-common method used to measure obesity in an individual. This simple measurement indicates obesity and morbid obesity in adults by measuring your waist. To find your waist circumference, wrap a tape measure around the area above your hip bone and below your rib cage.

For females, a waist circumference of 35 inches or greater is considered unhealthy. For men, a waist circumference of 40 inches or greater is considered unhealthy. There is not a classification chart or various ranges used with this method to determine obesity. Only the simple thresholds for men and women noted above apply.

How is BMI calculated and interpreted?

Calculation of BMI
BMI is calculated the same way for both adults and children. The calculation is based on the following formulas:

Measurement Units

Formula and Calculation

Kilograms and meters (or centimeters)

Formula: weight (kg) / [height (m)]2

With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters.

Example: Weight = 68 kg, Height = 165 cm (1.65 m)
Calculation: 68 ÷ (1.65)2 = 24.98

Pounds and inches

Formula: weight (lb) / [height (in)]2 x 703

Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.

Example: Weight = 150 lbs, Height = 5'5" (65")
Calculation: [150 ÷ (65)2] x 703 = 24.96

Interpretation of BMI for adults
For adults 20 years old and older, BMI is interpreted using standard weight status categories that are the same for all ages and for both men and women. For children and teens, on the other hand, the interpretation of BMI is both age- and sex-specific. For more information about interpretation for children and teens, visit Child and Teen BMI Calculator.

The standard weight status categories associated with BMI ranges for adults are shown in the following table.