Body Fat Scale Accuracy: How Do Scales Measure Body Fat

In addition to reporting your total body weight, a body fat scale takes into account your body composition or the percentage of body fat versus lean body mass (muscle, bone, etc.) that you have.

This can provide a more complete picture of your relative health risk and can be helpful when you are working to achieve body recomposition, which occurs when you lose body fat and gain muscle mass at the same time.

But, are body fat scales accurate? How do body fat scales measure body fat percentage?

In this article, we will discuss body fat scale accuracy, how body fat scales measure body fat percentage, and if there are better ways of measuring your body fat percentage than using a body fat scale.

We will cover: 

  • What Does a Body Fat Scale Measure?
  • How Do Scales Measure Body Fat Percentage?
  • Are Body Fat Scales Accurate?
  • What Affects Body Fat Scale Accuracy?

Let’s jump in!

A person stepping on to a body fat scale.

What Does a Body Fat Scale Measure?

Before we examine body fat scale accuracy, it is helpful to briefly cover what exactly body fat scales are measuring.

A body fat scale is designed to analyze your body fat percentage.

Body fat percentage refers to the relative amount of your body weight that is composed of fat tissue, or adipose, relative to lean body mass, which is basically any type of tissue other than body fat (bones, nerves, connective tissues, blood, organs, skin, etc.).

Your body fat percentage is one of the indicators of your overall health and risk of certain diseases.

According to the University of Pennsylvania, the average healthy adult body fat percentage range, regardless of age, is 15 to 20% for men and 20 to 25% for women. 

A body fat scale with handles.

How Do Scales Measure Body Fat Percentage?

So, how do body fat scales work? How is the body fat scale measuring your body fat percentage?

Body fat percentage is assessed on a body fat scale using a method known as bioelectrical impedance.

When you step on a body fat scale, you must remove your socks and shoes so that your bare feet come in contact with electrodes on the scale.

Bioelectrical impedance involves sending a safe, low-intensity electrical current through your body.

The principle behind this method of analyzing body fat percentage is that an electrical current travels at different rates through body fat and lean tissue because the resistance of these tissues to electrical current is not the same.

The electrical current emitted from the scale flows more easily through body tissues that contain a higher proportion of water (blood, muscle, urine) compared to tissues that are drier (fat and bone).

The body fat scale also takes into consideration things like your height, so the amount of time that it takes the current to travel up one side of your body and back down the other is fed into an algorithm that can determine your body fat percentage based on your size and the time it took the signal to travel through your body.

A doctor next to a person who measures their fat percentage with a body fat scale.

Essentially, the algorithm combines the bioelectric impedance measure (how quickly the electrical current travels through your body) with other inputs such as your total body weight, height, sex, age, and potentially even your fitness level, depending on the quality of the body fat scale. 

In general, the longer it takes the electrical current to travel from one side of your body up the leg and down the other, the higher your body fat percentage. 

This is because the electrical current will face more resistance to flow when traveling through drier tissue like fat compared to traveling through muscle tissue.

Higher-end body fat scales also have a handle with metal electrodes that is attached to the body fat scale. The handle also emits an electrical current that travels to your upper body.

The algorithm is then able to compute a more accurate body fat percentage using the segmented bioelectrical impedance from your upper body and lower body.

The general consensus is that body fat scale accuracy is markedly improved when the body fat scale does have an upper body and lower body portion because it is able to get a more complete picture of your true body fat distribution.

A person standing on a body fat scale.

Are Body Fat Scales Accurate?

Although many people put their full trust in the accuracy of a body fat scale, it’s important to remember that a tool is really only useful if it is accurate.

Unfortunately, many studies have suggested that body fat scale accuracy is not nearly as good as manufacturers may have us believing or that we would like to think.

Most studies have found that body fat scales underestimate your body fat percentage, and the accuracy of a body fat scale may also depend on your current body fat percentage and body size.

For example, one study compared the accuracy of three different body fat scales relative to the body fat percentage assessed via a DEXA scan (an accurate medical imaging technique to measure body composition). 

Results indicated that the body fat percentage measurements provided by the smart scales or body fat scales were not accurate in assessing body fat.

All three smart scales that were used underestimated body fat percentage as well as absolute fat mass. For example, one of the scales underestimated absolute fat mass by 4.4 kg (nearly 10 pounds!).

A scale that reads 76.4 kg.

The degree of discrepancy between the body fat scale measurement and the true body fat percentage as assessed by the DEXA scale was even greater for subjects who were obese or overweight. 

This indicates that body fat scale accuracy may be even poorer for those who carry excess weight.

Other research suggests that body fat scales may have a similar utility to that of body mass index (BMI) in that they can give a fairly good indication of body fat percentage for a population but not necessarily for certain individuals. 

Much in the way that BMI often categorizes fit athletes as overweight or obese due to their muscular build, body fat scale accuracy seems to be poor for overweight or obese individuals but may provide a decent body fat percentage estimate for large groups on average (populations vs individuals).

However, since most people using a body fat scale at home are only interested in measuring their own personal body fat percentage, this is not very helpful for the average home user. 

A doctor next to a person who measures their fat percentage with a body fat scale.

What Affects Body Fat Scale Accuracy?

There are numerous factors that can affect the accuracy of the body fat measurement on a body fat scale.

Body fat scales that solely use foot-to-foot analysis are generally less accurate than those that include a handle because the bioelectrical impedance misses large segments of your body as it is only assessing your lower body.

The algorithm the scale uses to estimate your total body fat percentage is extrapolated based on the body fat percentage in your lower body only. 

This can be highly inaccurate for certain individuals depending on your personal body fat distribution pattern.

For example, if you have a traditional “apple-shaped” body where you store most of your fat in your abdomen (a typical male-pattern body fat distribution), these types of body fat scales can underestimate your body fat percentage because the scale will assume that you are just as lean in your trunk in the upper body as you are in your legs.

On the other hand, if you have a “pear-shaped” body where you store more body fat in your hips and thighs, the body fat scale will overestimate your total body fat percentage because the algorithm will calculate your total body fat based on your build being just as “fatty“ in your upper body as you are in your hips and thighs. 

A person holding the handles of a body fat scale.

Most studies suggest that foot-to-foot body fat scale analysis often overestimates body fat percentage in lean individuals and underestimates body fat percentage in individuals who are overweight.

Additionally, the body fat scale accuracy is impacted by your hydration status. Higher fluid intake is reflected in a higher body fat percentage because of the resistance water provides to electrical currents.

Therefore, it’s important to try to be consistent in how you measure your body fat in terms of the time of day, hydration, and voiding of your bladder.

More accurate measurements of body fat can be obtained via DEXA scans and other medical/laboratory assessments.

For home assessments, you can monitor changes in body recomposition using circumference measurements or a device like ZOZOFIT, which is a body measurement system that uses 3D fit technology to help you monitor your changes in body composition or your body transformation over time. 

For more information on a healthy body fat percentage, check out our guide: What Is A Healthy Body Fat Percentage?

A doctor looking at a Dexa scan result.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, and contributes to several fitness, health, and running websites and publications. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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