Body Mass Index (BMI) is a number calculated from a person’s weight and height. Although it doesn’t directly measure body fat, it is a fairly reliable indicator of how much body fat an individual has. Additionally, it is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems. However, to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skin-fold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.
You can calculate BMI using the following two formulas:
- BMI = [Weight in Pounds/(Height in Inches) x (Height in Inches)] x 703
- BMI = Weight in Kilograms/(Height in Meters) x (Height in Meters)
For adults 20 years old and older, the Body Mass Index is interpreted using the same standards for all ages and for both men and women.
- A BMI of less than 18 means you are underweight
- A BMI of less than 18.5 indicates you are thin for your height
- A BMI between 18.6 and 24.9 indicates you are at a healthy weight.
- A BMI between 25 and 29.9 suggests you are overweight for your height
- A BMI of 30 or greater indicates obesity
The correlation between the Body Mass Index (BMI) number and body fatness is fairly strong; however the correlation varies by sex, race, and age. These variations include the following examples:
- At the same BMI, women tend to have more body fat than men.
- At the same BMI, older people, on average, tend to have more body fat than younger adults.
- Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness.
It is also important to remember that the Body Mass Index (BMI) is only one factor related to risk for disease. For assessing someone’s likelihood of developing overweight- or obesity-related diseases, you can read my post on Metabolic Syndrome.
As I said, the Body Mass Index (BMI) is not a direct measure of body fatness because it is calculated from an individual’s weight which includes both muscle and fat. As a result, some individuals may have a high Body Mass Index but do not have a high percentage of body fat. For example, highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness. The BMI ranges are based on the relationship between body weight and disease and death. Overweight and obese individuals are at increased risk for many diseases and health conditions, including the following:
- Dyslipidemia (for example, high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides)
- Type 2 diabetes
- Coronary heart disease
- Gallbladder disease
- Sleep apnea and respiratory problems
- Some cancers (endometrial, breast, and colon)