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How Does a Handheld BMI Machine Work?

author image Noelle Thompson
Noelle Thompson has extensive experience with health and scientific research, including in the biotechnology/pharmaceutical industry. She graduated from the University of California, Santa Barbara, with a B.S. in cell and developmental biology. Thompson then went on to earn a Ph.D. in biological chemistry, with an emphasis on stem cell biology, from the University of California, Irvine.
How Does a Handheld BMI Machine Work?
Person's feet on a scale with a tape measure around the ankles. Photo Credit NikolayShubin/iStock/Getty Images

BMI, or body mass index, is often used as a measure of your fitness. A high BMI can indicate being overweight or even being obese. Knowing your BMI is important in assessing your fitness. The handheld method gives rapid measurement of your BMI; however it is not always the most accurate reading. Other factors must be considered, including age, muscle content and gender.


BMI is a measure of the percentage of fat in your body. You can quickly calculate your BMI, by taking your weight in pounds, dividing it by the square of your height in inches and multiplying this number by 703, to compensate for the metric conversion. This will give you your BMI. According to the Centers for Disease Control and Prevention, a BMI less than 18.5 is considered underweight, while a BMI over 25.0 is considered overweight. A BMI over 30.0 is considered to be obese and should be discussed with your doctor immediately.

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Handheld Method

Measuring BMI with a handheld machine has become very popular, since it is fast and easy. The machine sends a small electrical current through your body and measures the resistance the current encounters. The current travels easily through water, but does not travel through fat. The machine measures the resistance the current experiences as it travels through your body; the higher the resistance, the more fat there is. This is considered to be a more accurate method than simply using your height and weight, which does not take into consideration other factors that can affect your fat composition, such as age, muscle content and gender.


The accuracy of the handheld BMI machines, compared with other methods, is questionable. While this method is considered to be more accurate than the simple weight versus height calculation, it is not as accurate as other methods, such as underwater weighing. The handheld method tends to be more readily available than these other methods and requires less complicated equipment; however it has the disadvantage of variation from machine to machine. According to an October 2007 article by Ursula G. Kyle and colleagues in the journal “Clinical Nutrition,” a lack of standardization exists between these machines, as well as a lack of proper quality control concerning their calibration. These machines also assume that you are properly hydrated. The current travels through your body’s fat content, as well as water, so if you are dehydrated or overly hydrated, the reading will not be accurate.

Other Considerations

Several factors should be taken into consideration when calculating your BMI. Age can be an important factor, since older people tend to have more body fat at the same BMI as younger people. Similarly, women tend to have higher body fat content at the same BMI as men. Muscle content also affects BMI. Athletes tend to have high BMI readings, because they have high muscle content, which weighs more than fat. In this situation, the handheld method is much more accurate than the weight versus height calculation, because it measures the actual fat content.

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