Body mass index is the ratio of your weight in kilograms divided by your height in meters squared. BMI provides an estimation of your body fat level, but is in no way diagnostic. It helps healthcare providers evaluate whether you may be at risk of chronic disease due to carrying excess fat. Although BMI is an acceptable measure for large parts of the population, it doesn't account for differences in age, activity level, gender, body composition or frame size. BMI is a set equation, so you can't adjust it for the size of your skeleton, or frame size, which might also reduce its usefulness.
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Body Mass Index Basics
To figure your BMI using pounds and inches, divide your weight in pounds by your height in inches squared; then multiply the number by 703, a conversion factor.
BMI = weight in pounds / [height in inches x height in inches] x 703.
Calculating your BMI results in a number that can help a healthcare provider evaluate whether you're at a healthy weight. If your BMI is below 18.5, you're considered underweight. If your BMI is between 18.5 and 24.9, you probably have a normal amount of body fat. A BMI of 25 to 29.9 is overweight, and a BMI above 30 is obese. BMI is convenient, noninvasive and easy to compute, and, although it's not a direct measure of body fat, it can be an effective screening tool.
Frame Size Considered
Determine your frame size by measuring the circumference of your wrist. For women shorter than 5 foot, 2 inches, a measurement less than than 5.5 inches indicates a small frame; a circumference of 5 to 5.75 inches indicates a medium frame; and a large frame comes with a wrist circumference of greater than 5.75 inches. For a woman who is between 5 foot, 2 inches and 5 foot, 5 inches, a wrist size of less than 6 inches indicates a small frame and one greater than 6.25 inches is large. Women taller than 5 foot, 5 inches with a small frame have a wrist circumference less than 6.2 inches, and those with a large frame have one greater than 6.5 inches. In men taller than 5 foot, 5 inches, a wrist size below 6.5 inches indicates a small frame, and one greater than 7.5 inches indicates a large frame size.
A large frame indicates a larger and heavier bone structure. This could cause you to weigh more than average for your height, but not necessarily due to excess fat, so a range of weights can still be consistent with a normal BMI. If you don't have too much fat but have a larger frame, you may weigh in the upper end of the weight range for your height, while you'd likely fall into the lower part of the range if your frame is small. For example, a 5-foot, 11-inch-tall person could weigh anywhere between 136 and 178 pounds and have a normal BMI.
Evaluating BMI in Terms of Frame Size
Even if your weight is normal but you have a body fat level of 20 percent as a man or 30 percent as a woman, you could be at much higher risk of diseases such as heart disease and type 2 diabetes. BMI is just one of many evaluations your doctor could perform when analyzing the health of your weight. Family history and lifestyle questionnaires, a physical exam, blood pressure and cholesterol checks and blood tests help determine if you're healthy for your height and frame size, or if you're at an increased risk of disease.
If you have a small frame size, but are in the upper range of normal BMI for your height, it could indicate that you have too much body fat. Additional testing, as well as more in-depth body fat analyses, such as body fat caliper tests, bone density scans and air displacement weighing, may better evaluate your health.
BMI and Muscular Builds
BMI is less accurate for people who have a large frame or muscular body. For example, you may weigh a lot for your frame size if you are extremely muscular because, pound for pound, muscle is more dense than fat and takes up less space. But an abundance of muscle does not pose a health risk. Regardless of your frame size, most doctors can visually assess whether it's an abundance of muscle, or fat, that's contributing to a high BMI.
For example, a 5-foot, 10-inch bodybuilder may weigh 200 pounds, but have just 9 percent body fat. His BMI would fall in the overweight range at 28.7, but his lean physique and active lifestyle puts at him at a low risk of disease despite his relatively high BMI.