Arm Circumference & BMI Comparison

When you can't determine the height of a patient, upper arm circumference can help calculate his body mass index. Body mass index, or BMI, gives a caregiver an estimation of a patient's body fat level. Knowing BMI influences the development of a treatment plan, especially when dealing with a person who is malnourished or obese.

Young woman flexing bicep in gym, close-up, side view
Measurements of your upper arm provide a rough correlation to BMI. (Image: Peter Cade/DigitalVision/Getty Images)

About Body Mass Index

Body mass index is usually figured by taking a person's weight in kilograms and dividing it by height in meters squared. A BMI that registers below 18.5 is considered underweight. A BMI between 18.5 and 24.9 is normal. When BMI goes above 25, a person is considered overweight, and above 30, obese. (ref4)

BMI is useful as a screening tool for conditions related to being under- or overweight, but it isn't diagnostic. It can help a doctor determine which type of dietary treatment a person under medical care may need and to check weight changes over time.

Using Arm Circumference to Determine BMI

In some cases, such as with a person in intensive care or one who is bedridden or in a wheelchair, height can't easily be obtained. Upper arm circumference roughly correlates with BMI in the average person.

A study of 44 patients found that people with an arm circumference below the 5th percentile and greater than the 15th percentile were more likely at risk of major health complications and early death, according to research published in a 2002 issue of Clinical Nutrition. For a person in long-term care, upper arm circumference measurements can also indicate changes in BMI. If a person's upper arm measurement changes by 10 percent, it's likely that BMI and weight have also changed by 10 percent.

How to Measure Upper Arm Circumference

Use a person's left arm to measure upper arm circumference. Ideally, the person is seated with the arm hanging loosely at his side. Bend the elbow to 90 degrees and find the midpoint between the bony part of the shoulder, known as the acromion, and the elbow, known as the olecranon. Measure around this midpoint using a tape measure pulled snug, but not tight.

A measure below 23.5 centimeters, or 9.25 inches, indicates that the person may be underweight or borderline underweight with a BMI of 20 or lower. An upper arm circumference of 32 centimeters, or 12.6 inches, indicates a BMI of 30 or greater, or obesity.

Interpretations of Upper Arm Circumference Measures

Upper arm circumference provides a general idea of BMI, but isn't definitive. Taking two measurements of upper arm circumference each time you measure and figuring according to the average of the two measures yields a more accurate reading.

One study, published in the Journal of Obstetrics and Gynecology in 2001, found a rough correlation between mid-arm circumference, or MAC, and BMI; the researchers used the equation BMI = MAC in centimeters ± 2. However, the false positive rate for overweight was 15 percent.

People who are extremely muscular may register with a large arm circumference and high BMI due to developed bicep muscles, but not have an excess of fat. For critically ill patients, though, upper arm circumference may be the only way to easily monitor health risks due to their size.

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