Bioelectrical Impedance (BIA): how it works and how to interpret it correctly

Bioimpedance scales are in almost every gym and many homes, but they're misused all the time. The "18.4% body fat" reading your device gives you can swing several points just because you showered beforehand or ate an hour ago. The good news: if you control the conditions, it's a very practical tracking tool.

What does this test measure?

Bioelectrical impedance analysis (BIA) sends a very low-intensity electrical current (imperceptible) through the body and measures the resistance it encounters. Lean tissue (muscle, with a lot of water) conducts the current better than fat tissue.

From that resistance, combined with weight, height, age, and sex, the device estimates:

• Body fat percentage
• Muscle mass / lean mass
• Total body water
• Sometimes, visceral fat and basal metabolic rate

Equipment needed

• A bioimpedance device (handheld, foot-to-foot, or full-body like InBody/Tanita)
• Ideally, controlled conditions: same time of day, same hydration status

Step-by-step protocol

1. Take the measurement fasted or at least 2-3 hours after the last meal
2. Avoid intense exercise in the 12 hours beforehand
3. Use the bathroom before the measurement (a full bladder alters the result)
4. Take the measurement barefoot, with clean and dry feet/hands on the device's electrodes
5. Stand still and don't talk throughout the measurement
6. Record the result and, if possible, the exact time of the measurement to repeat it under similar conditions

Common mistakes

• Measuring at very different times of day (morning vs. after eating give very different results)
• Measuring after training, showering, or drinking a lot of water/alcohol/caffeine
• Switching devices between measurements (each brand uses its own algorithms, so absolute values aren't comparable across devices)
• Comparing a single day's number without looking at the trend

How to interpret the result

Body fat % (BIA)	Men	Women
Low	<8%	<15%
Healthy	8-19%	15-28%
Moderately high	20-24%	29-33%
High	≥25%	≥34%

General reference ranges for adults. Consumer devices tend to over- or underestimate compared to lab methods, so the absolute value matters less than the trend.

Related tests

Bioimpedance pairs well with:

• Skinfold body composition test — to cross-check results with another method
• Basal metabolic rate test — many BIA devices already estimate this directly

Turn noise into a trend

The typical problem with bioimpedance is that a client weighs themselves one day on one scale and another day on a different one, or at different times of day, and panics seeing "I've gained 2 kg of fat in one day" (physiologically impossible). With Movalytics you can log each BIA measurement (weight, fat %, muscle mass, water) along with the date and conditions, and visualize the real trend by filtering out day-to-day noise, instead of obsessing over each isolated number.

Frequently asked questions

Is a home bioimpedance scale reliable? For tracking trends in the same person, with the same device and same conditions, yes. For comparing between people or as an absolute "lab-grade" value, not so much. Why does my body fat % vary so much from day to day? Mainly due to hydration: electrical current travels better through more body water, so being more or less hydrated alters the reading even though your actual body composition hasn't changed. BIA or skinfolds: which is better? Neither is "better" in absolute terms. Skinfolds depend more on the assessor's skill; BIA depends more on measurement conditions. Using both as cross-references usually gives a more complete picture.