Opens today at 08:00Locations →
MojeInBody.cz
Start with InBody
What we measure
InBody 970/970SBody compositionInBody InGripGrip strengthMax PulseRecovery and vascular conditionBPBIO750 monitorBlood pressure and pulse
Before and after measurement
How to prepareResults explainedOnline portal preview
PricingGift vouchersContact
← Back to the results guide

Bioimpedance: from signal to estimate

InBody does not directly measure kilograms of fat or muscle. It records weight and the electrical response of the body; water, fat-free mass, muscle and fat emerge from the subsequent model.

Where to find it: The impedance table by frequency and segment among the additional parameters.

Illustration of multi-frequency current paths through body segments

Measurement principle

Separate measured from derived

Start with resistance and reactance as the raw electrical signal. Then follow the chain from body water to modelled components, remembering that each additional layer adds assumptions.

  1. 011. The device measures electrical opposition across segments and frequenciesA weak alternating current passes between contact electrodes, and the device records resistance and reactance separately for the arms, trunk and legs.
  2. 022. Body composition emerges from the subsequent modelFrom the electrical response, the device first derives water distribution and only then other body components.
  3. 033. Measurement conditions affect the whole chainWhen fluid distribution or electrode contact changes, the input signal shifts and derived values can shift with it.

01

1. The device measures electrical opposition across segments and frequencies

A weak alternating current passes between contact electrodes, and the device records resistance and reactance separately for the arms, trunk and legs.

Resistance mainly relates to the conductive path through body fluids. Reactance captures the time shift at cell membranes. Together they form impedance; their relationship is linked to phase angle.

Multiple frequencies and segmental measurement add information about how the electrical response differs between body regions and between low and high frequencies. Impedance itself, however, is not yet a litre of water or a kilogram of muscle.

  • Measured: weight, resistance and reactance.
  • Calculated from the signal: phase angle and modelled limiting values.
  • An electrical number is neither good nor bad by itself without context.
Electrical signal
Impedance and reactance
Prediction model
Body-composition estimate
Teaching diagram — these are not your measured data.

Explained values

Impedance and reactanceBIAImpedance and reactance are the raw electrical data the device actually measures — every other value on the result sheet is calculated from them.Phase angleWBPAPhase angle is an electrical quantity in degrees describing the relationship between reactance and resistance measured as current passes through the body — a value for following a trend, not a grade for your health.Phase angle by segmentSegmental phase angle gives the phase angle separately for the right arm, left arm, trunk, right leg and left leg, at three frequencies: 5, 50 and 250 kHz.
Related guides: Bioimpedance / BIA: what InBody shows, what it does not and when it makes sense →Related guides: Phase angle on InBody: what it says and what it does not →

02

2. Body composition emerges from the subsequent model

From the electrical response, the device first derives water distribution and only then other body components.

The simplified chain is impedance → body water → fat-free mass → fat as the difference from body weight. Skeletal muscle mass and many other items are further model outputs, not separate direct measurements.

The manufacturer does not publish every equation, so the result cannot be independently recalculated from the raw values. An estimate can be useful and repeatable, but the precision of one number is not the same as the accuracy of a laboratory reference method.

  • Body water is the first interpretive layer above the electrical signal.
  • Fat-free mass is not a synonym for skeletal muscle mass.
  • Fat mass emerges in the model as a difference, not by directly scanning fat.

Explained values

Total body waterTBWTotal Body Water is the sum of the water inside and outside your cells in litres, and it is the quantity InBody derives the rest of body composition from.Intracellular waterICWIntracellular Water is the volume of water inside the cell membranes in litres, and the manual puts it at roughly 62 percent of total body water in a healthy person.Extracellular waterECWExtracellular Water is the volume of water outside the cells — in the interstitial space and in blood plasma — and the manual puts it at roughly 38 percent of total body water.Fat free massFFMFat Free Mass is everything except fat — the sum of body water, protein and minerals in kilograms.Muscle massSMMSkeletal Muscle Mass is an estimate of the amount of voluntarily controlled muscle in kilograms — the muscle between bones and joints that produces movement.Fat massBFMBody Fat Mass is the amount of fat in kilograms — an absolute figure, unlike percent body fat, which is only a share of weight.
Related guides: Bioimpedance / BIA: what InBody shows, what it does not and when it makes sense →Related guides: InBody vs DEXA, calipers and bioimpedance: what to expect from each method →

03

3. Measurement conditions affect the whole chain

When fluid distribution or electrode contact changes, the input signal shifts and derived values can shift with it.

For a trend, keep a similar time of day, interval from food, drink, training and sauna, and correct hand and foot contact with the electrodes. A short-term change need not represent a true tissue change.

A series of measurements on the same device under comparable conditions provides the most value. When comparing with DEXA, another BIA scale or anthropometry, remember that the methods use different physical inputs and models.

Explained values

Impedance and reactanceBIAImpedance and reactance are the raw electrical data the device actually measures — every other value on the result sheet is calculated from them.ECW RatioECW/TBWThe ECW Ratio states what share of total body water sits outside the cells; it describes distribution, not health status.WeightWTWeight is the only quantity InBody actually weighs; every other component of body composition is derived from it and from impedance.Fat free massFFMFat Free Mass is everything except fat — the sum of body water, protein and minerals in kilograms.
Related guides: Why InBody results change after food, training or alcohol →Related guides: InBody vs a smart scale: why the results are different →

Evidence used

The guide follows manufacturer documentation, consensus BIA principles and expert reviews of the method. It consistently separates the raw electrical signal, subsequent derivation and the resulting body-composition model.

Content reviewed · 2026-07-26

Explanations make the most sense alongside your own trend.

The public guide is open to everyone. The client portal adds your own value, history and the context of a specific measurement.

Book an InBody measurementHow to prepare for a measurement
Give InBody measurement as a giftWe'll send the gift voucher by e-mail right away - valid at any branch.For companiesA benefit for your employeesFor trainers and nutritionistsObjective data for your clients