What are body fat scales, and how accurate are they?

Share on Pinterest

Body fat scales use bioelectrical impedance analysis (BIA) to estimate the relative percentages of different tissues and substances within the body.

BIA sends a weak electrical impulse through the body. The impulse encounters varying levels of resistance or “impedance” from the different tissues and substances.

The scales then use a mathematical formula that incorporates the value for electrical resistance with information about a person’s age, height, and gender. People typically supply this information via a smartphone or other electronic device.

The mathematical formula then estimates the relative percentages of the following:

• total fat
• muscle
• water
• bone density

Fat offers more resistance than either muscle or water. As such, higher resistance values tend to result in calculations of a greater percentage of body fat.

Body fat scales provide a rough estimate of a person’s BFP. However, they are not very accurate.

A 2016 study found that the BIA method of measuring body composition is accurate when using standard, published mathematical formulas. However, the body fat scale that the researchers tested did not use accurate formulas to estimate BFP. Therefore, it produced inaccurate estimates.

Also in 2016, Consumer Reports conducted a test of six different body fat scales. First, the researchers used a lab-based machine called a Bod Pod to take accurate body fat measurements. They then compared these results with the data that they retrieved from the home-use BIA body fat scales.

In this study, the home-use body fat scales either overestimated or underestimated BFPs. The most accurate readings were off by about 21%, while the least accurate were off by about 34%.

The primary advantage of using body fat scales at home is convenience. The scales are easy and safe to use. Also, as long as the scales are consistent in their measurements — even if these are inaccurate — they can help the person roughly track the effectiveness of their exercise regimen or diet over time.

However, body fat scales are not an accurate method for estimating BFP. According to doctors, they typically overestimate or underestimate BFP by a large amount.

Home body fat scales also cannot show where a person’s body fat is stored. The location of body fat is an important consideration when assessing a person’s susceptibility to disease. For instance, a 2016 study found an association between increased abdominal fat and a higher risk of developing cardiovascular disease.

Body mass index (BMI) uses a person’s height and weight to get a general idea of whether their weight is likely to affect their health.

According to the Centers for Disease Control and Prevention (CDC), BMI is a good tool for screening whether a person is underweight or overweight. However, it does not measure body fat directly.

BMI also does not take into account a person’s muscle mass. Exercise can help a person lose fat and gain muscle. As muscle is denser than fat, a person’s body weight may actually increase as a result of exercise.

A person could use both BFP values and BMI values to help guide their dietary and exercise choices. If either value is higher than average, a person should see their doctor. Higher values could indicate an increased risk of cardiovascular disease.

Having a BMI or BFP below the recommended range also has health risks.

A person can use several different methods to estimate their BFP. Some are suitable for use at home or in a doctor’s office, but others must take place under tightly controlled laboratory conditions. Typically, the most expensive options provide the most accurate estimates.

At-home options

Some methods that people can use to estimate their body fat at home include:

Calipers

Calipers are tools that measure the distance between two sides of an object. A person can use calipers to measure the thickness of skin folds on different parts of the body. The accuracy of caliper measurements depends on the experience of the person using them.

Handheld BIA devices

Handheld BIA devices use electrical impulses to estimate BFP. As with step-on body fat scales, they are not very accurate.

Laboratory options

The following body fat measurement tools require special machinery and provide much more accurate results:

Bod Pod

A Bod Pod is a machine that calculates a person’s body density. It does this by combining two measures: a person’s mass or “weight” and their volume.

A person sits inside the Bod Pod, which uses highly accurate scales to measure their weight. Meanwhile, special sensors detect the volume of air that the person’s body displaces based on air pressure changes that occur in the Bod Pod. This value represents the person’s body volume.

The Bod Pod uses the weight and volume values to calculate the person’s body density. A technician can then use a mathematical formula to estimate the person’s BFP.

A Bod Pod requires specially trained operators, who tend only to use it in research laboratories.

Hydrostatic underwater weighing

Hydrostatic underwater weighing (HUW) calculates a person’s body density and body volume to estimate their body composition.

HUW works by comparing a person’s weight on dry land with their weight underwater. It also measures the volume of water that a person’s body displaces when they sit inside a tank of water.

The above values make it possible to determine the person’s body composition, including their BFP, based on the principle that fat is less dense than muscle.

As with the Bod Pod, HUW is typically only available in research laboratories and university settings.

Body fat scales are not very accurate at estimating a person’s body fat percentage. However, they may provide a rough idea of how much fat a person has in their body.

Together, body fat scales and BMI can give a general indication of a person’s health. If either value is high, a person should see their doctor for a health check. The doctor can also provide advice on diet and exercise.