It's a good idea to know your BMI, but keep in mind that it's a tool, not a diagnosis. It doesn't take your lifestyle, family history or body characteristics into account. Only your doctor or a trained fitness professional can accurately assess the amount of body fat you have.
Identification
BMI stands for body mass index, a statistical representation of how much body fat you're likely to have based on your height and weight. According to the Centers for Disease Control and Prevention (CDC), your BMI is a comparison of your height and weight to that of the general public. It doesn't necessarily indicate whether you are overweight or underweight, but it can indicate potential problems that need further investigation. Your doctor might calculate your BMI while assessing your risk for weight-related conditions such as heart disease and diabetes.
BMI Calculation
To calculate your BMI, take your weight in pounds and divide it by your height in inches squared. Multiply the result by 703 to get your BMI. Based on your result, you'll fall into one of four categories. A result below 18.5 indicates you're statistically underweight. A result of 18.5 through 24.9 is considered normal. Overweight scores fall between 25 and 29.9. If you're obese, you'll likely score a 30 or above.
BMI & Body Composition
Your BMI might not accurately represent your body's amount of fat. The University of Montana at Bozeman points out that two men can share the same height and weight and thus the same BMI; however, if one man is a well-trained athlete with a large muscle mass and the other man is not physically fit, they do not have the same amount of body fat nor the same risk level for becoming obese. The CDC also points out that a man and woman might share the same height, weight and BMI, but the woman will likely have more body fat than the man. Similarly, older people usually have less muscle and more fat than younger people.
Body Composition Types
According to UCLA's Student Nutrition Action Committee, there are three basic body types with genetic characteristics that determine how a person stores fat. Tall, thin ectomorphs have trouble gaining weight, usually resulting in a low BMI regardless of their health. Short, muscular mesomorphs have a hard time losing weight but are often strong and powerful; their BMIs might incorrectly indicate an overweight status. Endomorphs carry fat around their middles and have trouble losing it because their metabolisms are built to hold onto that extra weight through periods of dieting, often skewing a BMI calculation.
Considerations
All three basic body types can achieve a healthy weight that might not be reflected in their BMIs. According to UCLA's SNAC, 80 percent of Division I college football linebackers are considered morbidly obese despite their relatively low body fat and dense muscles and bones. If you're worried about a BMI that seems high for your fitness level, talk to your doctor or fitness professional. She can arrange for a more accurate measurement of your body fat using a skinfold thickness measurement.
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