# Bioimpedance: the smart scale (BIA)

The technique inside every body-composition scale: a small, harmless current is passed through you, the resistance it meets is measured, and from that and your height, weight, age and sex the scale estimates how much of you is water, lean tissue and fat.

Type: technique · Domain: Body Composition · Accuracy: Estimate · Importance: Core

Also called: bioelectrical impedance analysis, smart scale, body fat scale, body composition scale

## In one sentence

The technique inside every body-composition scale: a small, harmless current is passed through you, the resistance it meets is measured, and from that and your height, weight, age and sex the scale estimates how much of you is water, lean tissue and fat.

## How it works

Water conducts electricity well and fat conducts it poorly, so the resistance a body offers depends mostly on how much water it holds; water sits almost entirely in lean tissue, at a near-constant 73 percent of it, so total body water gives lean mass, and lean mass subtracted from weight gives fat (Kyle 2004). Everything on the scale screen descends from that one measurement and that chain of assumptions, which is why hydration moves every number at once.

## Foot-to-foot and hand-to-foot

A plain scale sends the current up one leg and down the other, so it measures the legs and guesses the rest from them; a scale with hand electrodes, or a five-segment scale, measures the arms, legs and trunk separately and guesses less. In 106 adults measured against MRI and DEXA, the hand-to-foot device had the narrowest limits of agreement for body fat (6.6 points under to 4.6 over) and no systematic bias, while the foot-to-foot devices were biased and ran as wide as 14.5 under to 8.6 over (Bosy-Westphal 2008). The VT Longevity Pro Scale is a five-segment device, which is why it can report each arm, each leg and the trunk, and why VT can compute an appendicular muscle index from it.

## What moves the reading

Hydration, food and drink in the last few hours, exercise and sauna, skin temperature, dry skin at the electrodes, a full bowel across the trunk, the menstrual cycle, and standing still or not; the review that set the clinical guidance lists them (Kyle 2004). The maker's equation adds a personal bias that stays roughly constant for you, which is what the DEXA correction removes.

## What it is good at

Following itself. Three low-cost devices repeated their lean-mass estimates with agreement of 0.92 to 0.99 between sessions (Vasold 2019), so the trend on one scale, measured the same way every morning, is real even when the level is off by several points. That is the whole basis of VT's use of the scale: calibrate the level once with DEXA, read the trend every day.

## How Vitality Tracker uses it

Weight and body fat from the daily reading feed the energy engine and the Vitality Score; the full reading (fat, lean, muscle, water, bone, visceral index, the five segments) is stored with each value marked as measured, derived, corrected or typed; every scale keeps its own series; and the morning routine in [*How to use a bioimpedance scale*](https://vitalitytracker.app/knowledge/how-to-use-a-bioimpedance-scale) is what makes the series worth reading.

## How far to trust the number

Estimate for the level, trend-grade for the direction on one device. Never compare two scales.

## Sources

1. Kyle UG, Bosaeus I, De Lorenzo AD, et al. Bioelectrical impedance analysis, part I: review of principles and methods. Clin Nutr 2004;23:1226–43. [doi:10.1016/j.clnu.2004.06.004](https://doi.org/10.1016/j.clnu.2004.06.004)
2. Bosy-Westphal A, et al. Accuracy of bioelectrical impedance consumer devices for measurement of body composition in comparison to whole body magnetic resonance imaging and dual X-ray absorptiometry. Obes Facts 2008;1:319–24. [doi:10.1159/000176061](https://doi.org/10.1159/000176061)
3. Vasold KL, et al. Reliability and validity of commercially available low-cost bioelectrical impedance analysis. Int J Sport Nutr Exerc Metab 2019;29:406–10. [doi:10.1123/ijsnem.2018-0283](https://doi.org/10.1123/ijsnem.2018-0283).

## Related

- [How to use a bioimpedance scale](https://vitalitytracker.app/knowledge/how-to-use-a-bioimpedance-scale)
- [Body fat](https://vitalitytracker.app/knowledge/body-fat)
- [Body water and water weight](https://vitalitytracker.app/knowledge/body-water)
- [DEXA](https://vitalitytracker.app/knowledge/dexa)
- [Why your weight and body fat wobble](https://vitalitytracker.app/knowledge/why-weight-and-body-fat-wobble)
- [Why VT asks about bowel movements](https://vitalitytracker.app/knowledge/why-vt-asks-about-bowel-movements)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/bioimpedance