As our smart devices get smarter, there are more options than ever for monitoring our health from home. 

Many home smart scales come equipped with an array of features, measurement options, and app integrations. This can leave us uncertain about which scale is the best option, which metrics matter the most, and the degree of accuracy these scales can offer. 

When it comes to body fat, many home scales use a process called bioelectrical impedance analysis – or BEI analysis – to estimate body fat percentage. If you’re curious about how to measure body fat percentage at home, this can be a great place to start. But exactly how accurate are body fat scales?

In this article, we’ll consider how BEI scales estimate your body fat, how accurate this body fat reading is, and what some alternatives are if you’re looking to accurately measure your body fat percentage and gain additional insights into your health.

What does a body fat scale measure?

A body fat scale, or body composition scale, is a scale that measures body composition in addition to weight. 

Popular smart scales from brands such as Garmin, Renpho, Withings, Fitbit and others allow body composition metrics to be measured from home, often synching with an app for easy monitoring. 

These scales use a variety of language to describe the measurements they offer, and you might see data points such as:

  • Body weight 
  • BMI
  • Body fat percentage
  • Fat mass
  • Muscle mass
  • Skeletal muscle 
  • Water content
  • Visceral fat
  • Metabolic age
  • And more 

However, it’s important to recognise that many of these data points are estimates – not measurements. This has a big impact on how accurate body fat scales are, and how their accuracy can vary from person to person. To understand why, let’s take a look at how body fat scales work. 

How do body fat scales work?

Body fat scales work via a process called bioelectrical impedance, which measures the resistance met by an electrical current as it travels through the body. 

When you stand on a smart scale, a low-level electrical current is passed up one side of your body and down the other side of your body. The amount of time it takes for the current to pass through depends on the resistance that is met in your body. The level of resistance depends on the water content of the tissue the current is passing through:   

  • A low resistance → a high water content
  • A high resistance → a low water content 

Different tissues in the body (such as bone, fat, and muscle) contain different proportions of water, measuring water content can help provide an estimate of body composition. For example, fat has a significantly lower water content than muscle. 

From this, your smart scale offers you an estimate of metrics such as body fat mass, muscle mass, and water. These metrics are based on average data – so exactly how accurate the readings are will depend on how similar you are to the population that was originally used to generate the data.  

Metrics a body fat scale can’t measure 

While a body composition scale can offer you an accurate measurement of some metrics (such as body weight) and estimates of others (such as body fat), there are some metrics which a home scale simply can’t measure. 

Visceral fat (the “hidden” fat that surrounds our organs) and bone density (the density of minerals in our bones) are two metrics which can’t be measured using a BEI-equipped scale. These metrics can only be properly measured using a three-component analysis of body composition such as is offered by a DEXA scan

How accurate are body fat scales?

When we talk about how accurate body fat scales are, it’s important to recognise that their accuracy is variable, and depends on factors such as:

  • The type of scale (two-point or four-point)
  • Hydration level
  • Gender 
  • Body fat distribution 
  • Activity level & training 

Research has shown that smart scales are considerably less accurate than a DEXA scan, and – in the case of two-point BEI measurements – may overestimate body fat percentage in lean individuals as well as underestimate body fat percentage in individuals carrying more fat. 

Overall, while smart scales can accurately measure body weight, they don’t offer an accurate estimation of body composition. However, they can offer an accessible at-home method of monitoring general trends in body composition. 

If you are going to use a body fat scale, aim to: 

  • Use the scale at the same time every day
  • Use the scale first thing in the morning, before food or drink
  • Stay hydrated 
  • Avoid using the scale after caffeine or alcohol
  • Avoid using the scale directly after exercise 

If you’re looking to assess your body composition more closely, there are several health and fitness tests that can offer a more accurate choice. 

Alternatives to body fat scales 

DEXA scan

The gold standard option for assessing body fat, as well as for body composition assessment in general, is the DEXA scan

DEXA stands for dual-energy X-ray absorptiometry, which describes the process by which the scan functions – by passing two, low-energy X-ray beams through the body. This is a quick and safe process that uses a low level of radiation, with the scan itself taking approximately 4 minutes to complete. 

Unlike a body fat scale, a DEXA scan can provide an accurate measurement of your body fat, fat deposition and android/gynoid ratio, visceral fat, lean mass and muscle distribution, and bone mineral density (BMD).

You can read more about what a DEXA body composition scan shows here

Skinfold callipers

A skinfold test is an accessible method of measuring body fat using a piece of equipment called skinfold callipers. 

Callipers are typically used to take skinfold measurements from the following areas: 

  • Abdomen
  • Triceps
  • Quadriceps
  • Below the shoulder blade (subscapular)
  • Above the hip bone (suprailiac)
  • Mid-chest (pectoral)

After measuring each area 3 times, averages are calculated. These figures are then inserted into a body composition calculator to estimate overall body fat percentage. 

It’s worth noting that skinfold callipers don’t measure your body fat percentage directly – rather, they estimate it based on the amount of fat you have in certain areas of your body. While this can be a fairly accurate method, it can vary depending on where you tend to store the most fat, as well as factors such as hydration and the competency of the person taking the measurements. 

Ideally, measurements would be taken by a trained professional, and the same person would carry out any future measurements to ensure consistency. 

Research shows that congruence between skinfold calliper measurements and BEI measurements of body fat varies; some studies show high congruence, whilst others suggest that it varies depending on the population being tested. 

However, skinfold callipers – like BEI-enabled body fat scales – can’t offer you insight into your levels of visceral fat, lean mass, muscular imbalances, or bone density. For those metrics, as well as details about your body fat distribution, you’d need to receive a full DEXA scan. 

Get started with a DEXA scan at My Vital Metrics 

The best way to accurately assess your body fat and overall body composition is through a professional DEXA scan

At My Vital Metrics, our DEXA scan for body composition can offer you detailed data into the amount and distribution of body fat mass, visceral fat, muscle mass, bone mineral density, and more. To find out more about our body composition testing services, or to book your free fitness consultation, reach out to My Vital Metrics today!