A person's BMI, or body mass index, is calculated by using height and weight. The body mass index does not directly estimate a person's body fat percentage. According to the Centers for Disease Control and Prevention, BMI is a "screening tool to identify possible weight problems for adults." The BMI figure places a person in one of four categories: underweight, normal, overweight or obese. For older people, using height can be unreliable due to age-related conditions such as curvature of the spine and spinal compression. Arm span may be used since it is approximately equal to height, and is not affected by age.
Step 1
Stand with your back to a wall, heels together.
Step 2
Raise both arms out to the side, palms facing out.
Step 3
Measure your arm span, with the help of the second person. The measurement should be taken from the tip of the middle finger of your left hand to the tip of your middle finger of your right hand. Record the result in inches.
Step 4
Weigh yourself on a scale, preferably wearing minimal clothing. Record the result in pounds.
Step 5
Consult an online BMI calculator to determine your body mass index. One example is the online calculator from the National Heart, Lung and Blood Institute (see Resources for the link). For the height measurement, use your arm span measurement. Enter your weight and press "Compute BMI."
Step 6
Refer to a BMI chart. You also can use a body mass index chart to calculate your BMI. One example is the chart from the National Institutes of Health website (see Resources for the link). Using your arm span measurement in inches, find that number in the left column. Slide across until you find your weight in pounds, or the number that comes closest. Move to the top of this column; the bold number is your BMI.
Things You'll Need
- Tape measure
- Scale
- Second person
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