Body mass index, BMI, is an indirect measure of total body fat. It is based on both measures of height and weight and is a relevant measure for adult men and women in assessing the risk factors for developing obesity-related diseases. It is not the only useful measure, however, and it does have limitations. Body fat percentage is another way of measuring your total body fat. It is simply the percentage of fat your body contains; if you are a 150-pound person, for example, having 10 percent body fat this means you have 15 pounds fat and 135 pounds lean body mass which includes bones, muscles, organ tissues and blood.
Calculating the BMI
Calculation of a your BMI involves straightforward math. Your weight in pounds is multiplied by 703 and then divided by height in inches. This number is again divided by the your height in inches. For example, a person who is 5-feet, 4-inches-tall and weighs 125 pounds has a BMI calculated as follows:
5 feet, 4 inches = 64 inches
125 pounds x 703 = 87,875
87,875/64 = 1373.05
1373.05/64 =21.45 = BMI
The Meaning of BMI Numbers
After your BMI is calculated, its meaning can be derived from the standardized chart on which a BMI of less than 18.5 indicates you are underweight and a BMI of 18.5 to 24.9 indicates your weight falls within the "normal" category. Individuals with BMI numbers of 25 to 29.9 are considered overweight. BMIs of 30 to 30.9 or greater are considered obese, and people with a BMI of 40 or above are considered to be in the extremely obese category.
Assessing Your Risk
Too much fat on your body, particularly fat localized to at the waist, increases a your risk for health problems including high blood pressure, high blood cholesterol and diabetes. These in turn increases the risk for heart disease and stroke.
Too little body fat is also problematic. Fat is essential to bodily functions, regulating body temperature, protecting organs and tissues and energy storage.
Body Fat Percentage
Body fat percentage, similar to BMI, can also be calculated. While the most accurate way to calculate total body fat percentage is underwater weighing, there is a formula that can be used as guideline.
It should be noted that different body fat percentages between genders and at different ages will equal the same health risks, due to the fact that women require higher fat levels for safe pregnancy.
In general, women between the ages of 20 and 40 are considered in a healthy range if they have 19 percent to 26percent body fat, while for women older than 40, a percentage of body fat in the range of 23 percent to 30 percent is considered good to excellent.
Men between the ages of 20 and 40 are considered healthy with 10 percent to 20 percent body fat, while men older than age 40 who have a percentage of body fat in the 19 percent to 23 percent range are considered good to excellent.
Body fat percentages outside those ranges are fair to poor.
Calculating Body Fat Percentage
If you're a woman, five measurements are required to determine body fat percentage: total body weight, wrist measurement, waist measurement, hip, measurement and forearm measurement. The lean body mass is a weighted sum of all those measurements. Body fat weight is the total body weight minus the lean body mass. Body fat percentage is the body fat weight times 100 divided by the total body weight.
Body fat percentage in women is calculated as follows:
Factor 1: Total body weight x 0.732 + 8.987
Factor 2: Wrist measurement at fullest point / 3.140
Factor 3: Waist measurement at naval x 0.157
Factor 4: Hip measurement at fullest point x 0.249
Factor 5: Forearm measurement at fullest point x 0.434
Lean Body Mass: Factor 1 + Factor 2 - Factor 3 - Factor 4 + Factor 5
Body Fat Weight: Total bodyweight - Lean Body Mass
Body Fat Percentage: Body Fat Weight x 100 / total body weight
If you are a man, the formula is much simpler as only two measurements are needed. Wrist, hip and forearm measurements are not used.
Factor 1: Total body weight x 1.082 + 94.42
Factor 2: Waist measurement x 4.15
Lean Body Mass: Factor 1 - Factor 2
Body Fat Weight: Total bodyweight - Lean Body Mass
Body Fat Percentage: Body Fat Weight x 100 / total body weight
Limitations of BMI
BMI is a useful tool, but it does have some limitations. It may overestimate or underestimate body fat in certain specific populations of people.
For example, if you are a well-trained athletes who has dense muscle mass, you may have a high BMI score but very little body fat. For you, a measure of the waist circumference and a measurement of skin fold thickness is a more direct method of measuring body fat, and may be more meaningful.
Also, in the elderly and other people who have lost significant muscle mass, the BMI may be an underestimate of total body fat. Again, in these cases, more direct measures would be more meaningful.
In addition, BMI does not give you any information on the location of the body fat. Body fat that accumulates at the waist is associated with a high obesity-related risk for disease.
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