Your body mass index, otherwise known as BMI, is calculated by your weight and height measurements. Your BMI is not a direct indication of weight, but places your body into different categories such as: underweight, normal, overweight, or obese.
Since a persons arms span is approximately the same as their height, this measurement may be used as a fairly reliable means of measuring BMI.
Stand straight up with your head facing forward, and your arms straight out, parallel to the floor. This ensures the most accurate measurement of your arm span.
Measure (in inches) from left fingertip, across torso, ending at right fingertip. This is an approximate measure of your height.
Use a scale to weigh yourself, preferably in the morning, without clothing. If you have the time, do a weight measurement every day for three days. Total these three measurements and divide that number by three. This is your weight in pounds.
Calculate your BMI with the aid of numerous online calculators for your data, or the following formula: (weight in lbs x 703 ) / (height in inches)².
Check your results, which fall into one of four categories. A BMI below 18.5 indicates your body is underweight; 18.5 to 24.9 places your weight within normal range. A BMI of 25 to 29.9 may indicate that your body is over the recommended weight for your height; and a BMI over 30 places your weight into the obese category.