The Number on the Body-Fat Scale

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Weight Management · Foundations
Body mass index (BMI) cannot tell muscle from fat, so many people turn to body-fat percentage instead; but compared with the reference method, the error range of a body-fat scale reading typically spans 15–20 percentage points. Neither number alone answers whether you carry too much fat; read them together with waist size and other signs.

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It does not measure fat

EVIDENCEREVIEW 2

READ THE ORIGINAL

You step on, and it reads 22.4%. One decimal place: it looks like a measurement.

It is not a measurement. It is an inference.

A body-fat scale does exactly one thing. It passes a very small electric current through your feet (and your hands too, on models with handles) and measures how much resistance the current meets on its way through your body. That resistance is called impedance.

What it measures is not fat but how easily electricity travels. And how easily electricity travels through the body depends mainly on water: muscle, blood and organs hold a lot of water and conduct well; fat holds little water and conducts poorly. So the raw data the scale actually gets is a number closely tied to how much water is in your body.

Between that number and the 22.4% on the screen lie two more steps of inference — and fat itself is never measured, not even once.

Educational content, not medical advice — consult a clinician.

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The Number on the Body-Fat Scale

Story

Connected to · 1

contrast · 1

Weight Management · Foundations
Body mass index (BMI) cannot tell muscle from fat, so many people turn to body-fat percentage instead; but compared with the reference method, the error range of a body-fat scale reading typically spans 15–20 percentage points. Neither number alone answers whether you carry too much fat; read them together with waist size and other signs.

Read full story

Stop 1/5

It does not measure fat

EVIDENCEREVIEW 2

READ THE ORIGINAL

You step on, and it reads 22.4%. One decimal place: it looks like a measurement.

It is not a measurement. It is an inference.

A body-fat scale does exactly one thing. It passes a very small electric current through your feet (and your hands too, on models with handles) and measures how much resistance the current meets on its way through your body. That resistance is called impedance.

What it measures is not fat but how easily electricity travels. And how easily electricity travels through the body depends mainly on water: muscle, blood and organs hold a lot of water and conduct well; fat holds little water and conducts poorly. So the raw data the scale actually gets is a number closely tied to how much water is in your body.

Between that number and the 22.4% on the screen lie two more steps of inference — and fat itself is never measured, not even once.

Educational content, not medical advice — consult a clinician.