Biometric Scales: What Metrics Truly Matter?

Biometric scales measure the weight and proportions of body fat and muscle. These devices enable people to estimate their weight and body composition quickly at home. While the tools do not replace the advice of health care professionals, they can give people useful insight regarding weight, body fat, and lean muscle mass.

Key takeaways:
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    Biometric scales give a speedy assessment of weight, body fat percentage, and other important health factors.
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    These tools work by sending a small electrical current through the feet and measuring how fast the current travels through body tissues.
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    Factors including gender, ethnicity, obesity, posture, and hydration level can affect the accuracy of readings.

How biometric scales work

Biometric scales are easy to use at home. They work with sensors that get activated when you step on the scale. Using a non-invasive method called bioelectrical impedance analysis (BIA) or bioelectrical impedance (BEI), a weak electrical current travels through the feet, legs, and pelvis. The scale’s sensors measure the impedance, or resistance, to the current passing through the body.

The scale then assesses body fat and fat-free mass (FFM) based on how fast the current goes through body tissues. FFM includes bones, connective tissues, vital organs, and water. The lowest impedance values are used to calculate total body water, which determines FFM.

Fat contains less water than muscle, making it more resistant to electrical charges. This leads to higher impedance values and subsequent calculations of higher body fat percentages (BFP).

Biometric scales use a mathematical formula combining the impedance value with data about a person’s age, gender, and height. People enter this information into smartphones, smartwatches, or other fitness apps. The mathematical formula then calculates approximate percentages of total fat, water, muscle, and bone density. The smart device saves this data for tracking ease.

What metrics are important?

Many people depend on biometric scales to measure weight, body fat, muscle mass, protein mass, and metabolic rate. The most common readings include the following:

  • Total body weight
  • Body mass index (BMI)
  • Body fat percentage (BFP)
  • Fat mass
  • Fat-free mass (FFM)
  • Total body water
  • Basal metabolic rate

These figures can help render a reliable assessment of health and fitness progress over time.

Factors affecting scale accuracy

Biometric scales can provide approximate measurements for weight and body fat percentages. While these tools may calculate weight precisely, research shows that they might not accurately determine body composition. Several factors affect BIA computations, including the ones discussed below.


Accuracy depends on a set of assumptions about gender, body proportion, hydration, fat-free body density, training level, and demographic characteristics. These factors can vary broadly among multi-ethnic groups. According to a 2021 study, typical body composition estimates may not be valid for some ethnic populations.


Individuals with obesity may see underestimated fat mass results on biometric scales. These people have a higher percentage of FFM hydration due to expanded extracellular space in fat tissues. This skews the hydration factor used to compute FFM figures. The error grows with increasing body mass index (BMI).

Hydration level

Water intake increases the volume of the body. It also potentially lowers BIA by temporarily diluting ion concentrations in the blood. On the other hand, dehydration may lead to underestimated muscle and bone calculations.

Postural changes

A 2022 article reported that lying down, sitting, or standing for prolonged periods can redistribute body fluids. This can result in miscalculated estimates as well.

How to ensure accurate readings

Biometric measurements might be affected by foot size, foot width, and how the legs are bent while on the scale. Follow the manufacturer’s instructions as closely as possible and use these tips for more consistent readings:

  • Place the scale on a hard surface.
  • Sit or stand upright for at least 15 minutes prior to using a scale.
  • Maintain the same position on the scale with each use.
  • Weigh in at the same time of day.
  • Avoid bending legs while on the scale.

Considerations for using a biometric scale

The ease of weighing in on a biometric scale at home is perhaps its greatest advantage. However, it is only one useful tool for managing your health.

A biometric scale can estimate your BFP, but it doesn’t factor in how your body fat is distributed. For instance, a scale cannot pinpoint excess visceral fat in the abdomen or liver fat, either of which is associated with heart disease and diabetes.

Alternatives for measuring body composition

Other noninvasive methods may help complete your weight and body composition picture in addition to using a biometric scale. Some are inexpensive and easily accessible at home, while some require more costly professional testing at a facility.

Here are some alternatives that you could consider:

  • Body mass index (BMI)
  • Weight measurements
  • Body fat calipers
  • Handheld fat measurement tools
  • Bod Pod-Dual-energy X-ray absorptiometry (DEXA) scans
  • Hydrodensitometry (underwater) weight test

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