Bioelectrical Impedance Analysis (BIA) Explained: How It Works, Accuracy, and Limitations

How BIA Works: The Science

BIA devices pass a low-frequency alternating electrical current (typically 0.5–1 mA at 50 kHz) through body tissue. The device measures electrical impedance — the opposition to current flow — and uses this value with your age, sex, height, and weight to estimate body composition.

The underlying principle: lean tissue is approximately 73% water, making it highly conductive (low impedance). Adipose (fat) tissue contains very little water and resists electrical current (high impedance). By measuring how much resistance the current encounters, BIA estimates the proportion of fat vs lean tissue.

BIA devices range from simple bathroom scales with foot electrodes to sophisticated multi-frequency, 8-electrode clinical devices. Consumer scales (foot-to-foot) are the least accurate; professional 8-electrode devices (InBody, Seca) approach ±2–3% accuracy.

Types of BIA Devices

What Affects BIA Accuracy

BIA is highly sensitive to the following variables — any of which can shift your reading by 2–5% body fat:

• Hydration: The biggest factor. Even mild dehydration (1–2% of body weight) increases impedance, causing BIA to overestimate fat %. Being well-hydrated decreases impedance, causing it to underestimate fat %. Always measure at the same hydration state.
• Recent food intake: Food and fluids in your digestive system increase body water and mass, affecting readings. Measure fasted (before eating or drinking).
• Exercise: Recent exercise causes acute dehydration and electrolyte shifts. Measure at least 4 hours after exercise.
• Time of day: Body water distribution changes throughout the day. Measuring in the morning after bathroom use gives the most consistent baseline.
• Menstrual cycle: Water retention fluctuates by 0.5–2 kg throughout the cycle, directly affecting BIA readings. Women should measure at the same cycle phase each month.
• Skin temperature: Cold extremities increase impedance. Measure at room temperature, not after being outside in cold weather.

How to Get the Most Reliable BIA Readings

Follow this protocol consistently every time you measure:

1. Measure first thing in the morning, immediately after waking.
2. After using the bathroom (empty bladder and bowel).
3. Before eating or drinking.
4. At least 12 hours after your last large meal.
5. At least 4–8 hours after exercise.
6. Barefoot with clean, dry feet (for foot-electrode scales).
7. Always use the same scale — different devices give different readings.

With this protocol, BIA readings become meaningful as a trend indicator even if individual readings are imprecise. Month-over-month trends are more reliable than individual data points.

BIA vs Other Methods: When to Use Each

BIA is best used as a convenient trend tracker when you commit to the same protocol each time. It's not the best choice for absolute accuracy — for that, use DEXA or the Navy circumference method. The Body Fat Calculator (Navy method) is free, unaffected by hydration, and typically more consistent than consumer BIA scales.

If you already own a BIA scale, use it with the strict protocol above. If you're choosing a measurement method from scratch, the free Navy method calculator is more reliable for progress tracking than a consumer BIA scale. See the full comparison in DEXA Scan vs Body Fat Calculator.