Body mass index, or BMI, is a good guideline for keeping track of your weight. Your BMI value can be classified as underweight (less than 18.5), normal (18.5 to 24.9), overweight (25 to 29.9) or obesity (30 to 39.9).
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Since weighing more than you should increases your risk for health problems like heart disease, knowing your BMI can be an important part of taking care of yourself. If you've had an amputation, however, you need to account for the weight of the missing body part to calculate your BMI. Anthropologists have figured out a good way for amputees to calculate their BMI.
Weigh yourself and write down your weight.
Look up the percentage of body weight value for your type of amputation (see Tips). For example, if one of a man's legs was amputated, the missing leg would be 16 percent of his total body weight.
Move the decimal point 2 places to the left in the number from Step 2. For example, the value is now 0.16.
Subtract the result of Step 3 from 1. For example, 1 - 0.16= 0.84.
Divide the current weight by the result from step 4. For example, if a man weighs 160 lbs, then 160 ÷ 0.84= 194.48. This is his estimated total weight without the amputation.
Multiply the estimated total weight by 703. For example, 194.48 X 703 to get 136719.44.
Find the height in inches. For example, if a man is 6 feet 2 inches, he will multiply (6 feet) X (12 inches in a foot)= 72 inches, plus 2 is 74 inches.
Find the square of the height in inches. For example, 74 X 74= 5,476.
Divide the result from Step 6 by the result from Step 8. In the example, 136719.44 ÷ 5476= 24.97. This value is the man's BMI.
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The Journal of the American Dietetic Association gives the following percentage of body weight values for some types of amputation: All 4 limbs amputated = 50 percent; entire arm = 5 percent; forearm and hand = 2.3 percent; just one hand = 0.7 percent; entire leg = 16 percent; lower leg and foot = 5.9 percent; just one foot = 1.5 percent.
The formula for calculating BMI using metric measurements is (weight in kilograms) ÷ (height in meters)²
Since BMI is based only on height and weight, it is not accurate for everyone. For example, a very muscular person can have a high BMI without having overweight, since all of the extra muscle adds a lot of pounds to the frame.