BIA (Bioelectrical Impedance) Calculator

BIA: Bioelectrical Impedance Analysis

BIA (Bioelectrical Impedance Analysis) runs a low-intensity electrical current through the body, usually at 50 kHz, and reads back resistance (R) and reactance (Xc). Lean tissue (water + electrolytes) conducts well while fat resists, so the impedance gives you an estimate of body composition. The classic equation starts with lean mass, LBM ≈ (0.756·H² / Z) + constants, and from there body fat % = (weight − LBM) / weight × 100. Worked through: a man at 70 kg, 170 cm, Z = 500 Ω lands at LBM ≈ 56 kg, so fat ≈ 20%. ACE ranges (men) run essential 2–5%, athlete 6–13%, fitness 14–17%, acceptable 18–24%, obesity ≥25%. Hydration moves the numbers a lot, which is why the standard protocol asks for 4 h fasting, no alcohol for 12 h and no exercise for 12 h. DEXA is still the gold standard at ±1–2%, while BIA swings 3–5% from one brand to the next (Tanita, InBody, Omron).

Applications and context

You'll find BIA all over clinical nutrition, gyms, sports clinics and elite athletics, wherever someone needs to follow lean mass and fat across weeks and months. Nothing is invasive, it takes under a minute, and it costs a fraction of DEXA or hydrostatic weighing. Stick to the same device and the same protocol and BIA tracks change reliably. When you need the absolute number to be right, reach for DEXA instead.

FAQ

Why does my body fat reading change from day to day? Impedance reacts to hydration, your last meal, alcohol, exercise, even how warm your skin is. Measure under the same conditions every time: morning, fasted, after voiding.

Are home scales (Tanita, Omron) accurate? They're good for spotting trends, though they can sit 3–5% off DEXA. Keep to the same scale, the same time of day and the same protocol, and watch the curve rather than the absolute number.

BIA or DEXA? For a single accurate reading, DEXA is the gold standard at ±1–2%. For checking in often, BIA wins on cost and how easy it is to get to.