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%

    Frequently Asked Questions

    How accurate are BIA scales?

    Consumer BIA scales can be off by 3-8% for body fat. They are most useful for tracking trends over time rather than absolute accuracy.

    What affects BIA accuracy?

    Hydration level, recent exercise, meals, and time of day all affect readings. Measure at the same time under the same conditions for best comparisons.

    Is BIA better than BMI?

    BIA provides more information than BMI because it distinguishes between fat and lean tissue. However, neither is as accurate as DEXA scanning.

    Ready to run the numbers?

    Open Bioimpedance Body Composition Estimator