Bioimpedance (BIA)
Bioimpedance analysis passes a small electrical current through the body and measures the resistance it meets. Because lean tissue conducts well and fat does not, the device infers total body water, then estimates fat-free mass through an equation. It measures water, not fat.
Bioimpedance: what is it?
Every smart scale, gym handgrip device and clinic-grade eight-electrode machine works on the same principle: fat is a poor conductor, lean tissue full of water and electrolytes is a good one, so the resistance the current meets tells you roughly how much water is in the body. From there an equation, built on a reference population, converts water into fat-free mass and fat-free mass into a body fat percentage. Every step after the resistance measurement is an assumption, and that is the whole story of bioimpedance.
The three device types measure different things
Not all bioimpedance is the same, and the difference is about which parts of the body the current actually travels through. A client stepping barefoot on a bathroom scale is not getting the same measurement as a client on an eight-electrode segmental machine, and the two are not comparable.
| Device type | Current path | What that means |
|---|---|---|
| Foot-to-foot (smart scale) | Up one leg and down the other | Reads the lower body. Trunk and arms are largely inferred |
| Hand-to-hand (grip device) | Across the upper body | Reads the upper body. Legs are largely inferred |
| Eight-electrode segmental | Hands and feet, multiple paths | Measures segments separately. The most informative of the three |
| Multi-frequency | Several frequencies through the same paths | Can separate intra- and extracellular water. Usually clinic equipment |
Why the same client gets different numbers on the same day
Bioimpedance is measuring hydration, so anything that shifts hydration shifts the result. A meal, a glass of water, a training session, alcohol the night before, cold hands and feet, skin lotion, and the menstrual cycle all move the reading, and some of them move it enough to swamp weeks of real progress.
This is not a flaw in the device so much as a description of what it does. A client who weighs in dehydrated after a morning run will read as higher body fat, not because they gained fat but because they lost water and the equation has less lean tissue to work with. Fix the conditions and the noise drops dramatically.
A standardisation protocol worth enforcing
If you are going to use bioimpedance at all, the protocol is the whole method. Write it down, give it to the client, and hold them to it every single time. A measurement taken outside the protocol should be discarded, not recorded.
| Condition | Rule |
|---|---|
| Food and drink | Fasted, at least four hours since the last meal |
| Bladder | Empty, immediately before measuring |
| Training | None in the previous twelve hours |
| Alcohol | None in the previous twenty-four hours |
| Time of day | Same time every measurement, morning preferred |
| Skin | Clean and dry, no lotion or cream on hands or feet |
| Menstrual cycle | Same phase each time where applicable |
| Device | The same unit, every time, no exceptions |
How to use it without overselling it
Bioimpedance is cheap, instant, needs no skill from the tester and involves no physical contact, which makes it genuinely practical for clients who would decline calipers. Under a fixed protocol it tracks direction reasonably well over a period of months.
What it cannot do is give a defensible absolute body fat percentage, and it should never be the number a client fixates on. Report the direction, pair it with waist circumference and photos, and if the estimate disagrees with the tape measure, trust the tape measure. Devices that pass a current are also generally contraindicated for clients with a pacemaker or other implanted electronic device, which is a manufacturer instruction to follow, not a judgement call.
Key takeaways
- Bioimpedance measures resistance to a current, infers body water, then estimates fat-free mass through an equation.
- Foot-to-foot, hand-to-hand and eight-electrode devices measure different parts of the body and are not comparable.
- Hydration, food, training, alcohol, skin temperature and cycle phase all move the result.
- Under a strict protocol it tracks direction; it never gives a defensible absolute percentage.
Frequently asked questions
How accurate are bioimpedance body fat readings?
As an absolute figure, only approximate, because the device measures body water and converts it through a population equation that may not fit your client. As a trend under a fixed protocol on the same device, it is usable. Disagreements of several percentage points with calipers or DEXA on the same person are normal.
Why does my smart scale give a different number every day?
Because it is tracking hydration more than fat. A meal, a hard session, alcohol, salt, cold feet or a different time of day all change the reading. Day-to-day swings are noise. Measure under identical conditions and read the average across a week or a month instead.
Can everyone use a bioimpedance device?
No. Manufacturers generally advise against use for people with a pacemaker or other implanted electronic device, and readings are unreliable during pregnancy or with significant fluid retention. Follow the device instructions rather than making the call yourself, and use girths and photos as the alternative.
Updated August 28, 2026
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