Bioimpedance Body Composition Estimator Formula

Understand the math behind the bioimpedance body composition estimator. Each variable explained with a worked example.

Use the Bioimpedance Body Composition Estimator

Formulas Used

Fat Mass

fat_mass_lbs = weight_lbs * body_fat_pct / 100

Lean Mass

lean_mass_lbs = weight_lbs * (1 - body_fat_pct / 100)

Estimated Muscle Mass

est_muscle_lbs = weight_lbs * (1 - body_fat_pct / 100) * 0.55

Estimated Body Water

est_water_pct = (1 - body_fat_pct / 100) * 73

Variables

VariableDescriptionDefault
weight_lbsBody Weight(lbs)180
body_fat_pctBIA Body Fat %(%)22
height_inchesHeight(inches)70
weight_kgDerived value= weight_lbs * 0.453592calculated

How It Works

Understanding BIA (Bioelectrical Impedance Analysis)

BIA scales send a small electrical current through your body. Fat tissue conducts less electricity than lean tissue, allowing the device to estimate body composition.

From BIA Body Fat % We Can Derive

  • Fat Mass = Weight x Body Fat %
  • Lean Mass = Weight x (1 - Body Fat %)
  • Muscle Mass ~ Lean Mass x 0.55 (skeletal muscle is about 55% of lean mass)
  • Body Water ~ Lean Mass is about 73% water

    • Worked Example

    A 180 lb male at 22% body fat per BIA scale.

    weight_lbs = 180body_fat_pct = 22height_inches = 70
    1. 01Fat mass = 180 x 0.22 = 39.6 lbs
    2. 02Lean mass = 180 x 0.78 = 140.4 lbs
    3. 03Est muscle = 140.4 x 0.55 = 77.2 lbs
    4. 04Est water = 0.78 x 73 = 56.9%

    Ready to run the numbers?

    Open Bioimpedance Body Composition Estimator