BMI: Understanding Your Body Mass Index
What you should know about calculating your BMI

Body Mass Index (BMI) is a number calculated from a person’s weight and height. The Box Mass Index is a reliable indicator of the level of fat in your body. BMI does not measure body fat directly, but research has shown that BMI correlates with other ways of measuring body fat such as underwater weighing. BMI is a relatively easy and inexpensive way of determining your relative health in weight. In fact, 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, but this alone is not enough to determine whether the person is at health risk. A health professional would need to conduct additional tests before coming to a diagnosis. Other tests might include evaluations of diet, exercise, family history, and skin fold thickness. 

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.

BMI is calculated the same way for both adults and children. The calculation is based on the following formulas: your weight in kilograms divided by height in meters squared. For inches and pounds, you should calculate your BMI by dividing your weight in pounds by height in inches squared and multiplying that by 703.

Interpretation of BMI for adults

Below 18.5 BMI is considered underweight; 18.5-24.9 is considered normal, 25.0-29.9 is considered overweight, and 30.0 and above is considered obese.

The overall correlation between BMI and body fatness is fairly strong. However, there are certain variations that must be taken into account according to sex, race, and age. For example, women tend to have more body fat than men even at the same BMI. Older people also have more body fat than younger adults at the same BMI. In addition, athletes may have a higher BMI due to more muscularity rather than increased body fatness.

It is also important to remember that BMI is only one factor related to risk for disease. For assessing someone’s likelihood of developing overweight- or obesity-related diseases, experts recommend looking at the waist circumference (since abdominal fat is a predictor of risk for obesity-related diseases) and other indications of health risks related to excess weight including high blood pressure and lack of physical activity.

Remember: BMI is not a direct measure of body fatness and other factors must be taken into account. It is possible for someone with a BMI of over 25 to be considered overweight due to special circumstances, such as increased muscularity.