What is bioelectrical impedance analysis?

How it works

Lean tissue holds a lot of water and conducts electricity well. Fat holds little and resists it. A BIA device passes a small current through your body, measures the resistance (impedance) and plugs it, along with your height, weight, age and sex, into an equation that predicts body water, fat-free mass and body fat.

Devices differ in where the current travels:

• Foot-to-foot: most bathroom smart scales.
• Hand-to-hand: handheld devices.
• Hand-to-foot (octapolar): gym and clinic analyzers with handles.

What the research found

• Consumer smart scales vs DEXA (2021). Three smart scales were compared against DEXA in hospital patients. Median error for body weight was 0 to 0.3 kg. For fat mass, the scales were off by a median of 2.2 to 4.4 kg, underestimating fat. The authors concluded smart scales are not accurate for body composition.
• Fifteen BIA devices vs a lab four-compartment model (2023). Every device was highly repeatable, with precision error of 0.0–0.49%. Single-reading accuracy varied widely: average error ranged from −3.5 to +11.7 percentage points depending on the device. Agreement was better for change over 12–16 weeks, and octapolar and some foot-to-foot models did best.

That pattern matters. BIA is often consistent but biased: it may give you the wrong number reliably.

Why hydration skews it

BIA estimates composition from body water, so anything that moves water moves the result:

• how hydrated you are
• a large or salty meal
• a high-carb day (glycogen binds water)
• a workout
• alcohol
• skin temperature

None of these change your fat mass, which is why the same scale can swing between morning and evening.

How to use a BIA scale

Measure first thing in the morning, after the bathroom and before food, drink or training. Use the same device, compare weekly averages rather than single days, and don’t compare numbers across different brands. For the bigger picture, see how BIA compares to DEXA, calipers and photos.