Bioelectrical impedance: what it is and how it works

By PounceFit Team · Updated on 2026-10-06

Technical content for health and fitness professionals. It reproduces formulas and protocols published in the scientific literature, with the sources at the end of the page. It is not a recommendation and does not replace the judgment of the professional who runs the assessment.

Bioelectrical impedance analysis (BIA) is a method that passes an electrical current through the body and measures the body's opposition to it, the impedance. Because the current is carried by water with electrolytes, and adult fat-free mass is about 73% water, an equation uses impedance, height, weight, sex and age to estimate fat-free mass; body fat is the rest of body weight (Kyle et al., 2004, part I). The result is an estimate that depends on the device, the equation and the measurement conditions.

Resistance and reactance

The body offers two kinds of opposition to the current (Kyle et al., 2004, part I):

• Resistance (R): comes from the fluids inside and outside the cells.
• Reactance (Xc): comes from cell membranes, which behave like small capacitors.

Impedance is the combination of the two. The frequency of the current changes its path: at low frequency, the current barely crosses the membranes and travels through the fluid outside the cells; at high frequency, it travels both inside and outside. At 50 kHz, the frequency used by most single-frequency devices, the current passes through both compartments, in proportions that vary from tissue to tissue (Kyle et al., 2004, part I).

From impedance to body composition

The basis is the cylinder model: the resistance of a conductor is proportional to its length and inversely proportional to its cross-sectional area. It follows that the conductor's volume is proportional to its length squared divided by resistance. Since height is easier to measure than the length of the current path, devices use the index height² ÷ R (Kyle et al., 2004, part I).

The body is not a cylinder, so the relationship is empirical: a regression equation, built in a group of people also measured by a reference method, links the index to fat-free mass (Kyle et al., 2004, part I).

One published equation is that of Kyle et al. (2001), built in Geneva with 343 healthy white adults aged 20 to 94, with a BMI of 17 to 33.8, against DXA:

Fat-free mass (kg) = −4.104 + 0.518 × height² ÷ R + 0.231 × weight + 0.130 × Xc + 4.229 × sex

Height in cm, R and Xc in ohms at 50 kHz, weight in kg; sex = 1 for men and 0 for women. The standard error of the estimate was 1.72 kg. With fat-free mass, body fat follows by difference: fat (kg) = weight − fat-free mass.

Each equation holds for the population and device type in which it was built. ESPEN calls for an equation validated for the age, sex and ethnicity of the person assessed (Kyle et al., 2004, part II).

Device types

According to the ESPEN review (Kyle et al., 2004, part I):

• Single frequency (50 kHz): the research standard places electrodes on the hand and foot, with the person lying down. Some devices use other positions, such as foot-to-foot (scales) or hand-to-hand.
• Multi-frequency: uses impedances at several frequencies, from 0 to 500 kHz, in empirical equations to estimate fat-free mass, total body water and the water inside and outside the cells.
• Spectroscopy: uses mathematical models, rather than regression alone, to relate resistance to body fluids.
• Segmental: measures arms, legs and trunk separately.

Home scales come in two main formats: feet only (four electrodes) or feet and hands (eight electrodes). In the study by Siedler et al. (2023), with 15 devices, performance varied widely between models. See are bioimpedance scales accurate?

What affects the measurement

ESPEN lists the precautions for measurement (Kyle et al., 2004, part II):

• Height and weight measured on the spot, not reported by the person.
• Fasting and no alcohol beforehand, empty bladder and no exercise in the previous 48 hours (with shorter fasting periods accepted in clinical practice).
• Repeated measurements at the same time of day.
• Clean skin at the electrode sites.

Eating or drinking can lower impedance by 4 to 15 ohms over the following 2 to 4 hours, and right after exercise resistance drops by about 3%. At extreme BMI or with abnormal hydration, ESPEN does not recommend routine use without specific validation (Kyle et al., 2004, part II).

Bioimpedance in PounceFit

In PounceFit, the body assessment software, bioimpedance comes from a bioimpedance scale connected over Bluetooth. The app receives the weight and the impedance in ohms, calculates body composition with the height, sex and age of the client on file, and keeps the raw impedance so it can recalculate if the professional changes formula. The recognized scales are listed under compatible scales.

To compare with the other field method, see bioimpedance vs skinfolds.