BTL

The BodPod (Air Displacement)

Air Displacement Plethysmography reviewed. Is the BodPod more accurate than DEXA?

The Dry Alternative to Hydrostatic Weighing

For decades, the gold standard for measuring body density was Hydrostatic (Underwater) Weighing. However, submerging subjects in water and forcing them to expel all air from their lungs is highly impractical.

Air Displacement Plethysmography (ADP), commonly known by the brand name BodPod, solves this problem by using air instead of water.

How the BodPod Works

The BodPod uses the exact same physics as underwater weighing (Archimedes' Principle), but measures the volume of air displaced by the subject's body.

  1. Volume Calibration: The machine measures the exact volume of air inside the empty pod.
  2. Subject Measurement: The subject sits inside the pod. The machine measures the new volume of air.
  3. Body Volume Calculation: Empty Pod Volume - Occupied Pod Volume = Subject's Body Volume.
  4. Density Calculation: The subject's scale weight is divided by their Body Volume to calculate Body Density (Db).
  5. Body Fat Estimation: The Body Density is run through a standard formula (like Siri or Brozek) to estimate body fat percentage.

Strict Clothing Protocols

Because the BodPod measures air displacement with extreme sensitivity, even the air trapped in your clothing or hair will ruin the reading.

Subjects must wear:

  • Tight-fitting spandex (Lycra) compression shorts or a swimsuit. (Loose clothing traps air, making you seem "larger" and artificially increasing your body fat result).
  • A tight swim cap. (Hair traps massive amounts of air).

If you test in a BodPod wearing standard gym shorts and a t-shirt, the data is entirely useless.

BodPod vs DEXA

Accuracy: The BodPod has a margin of error of roughly ±2.0% to ±3.0%. It is slightly less accurate than a DEXA scan (±1.5% to ±2.5%).

The Major Flaw: Like hydrostatic weighing, the BodPod relies on the Siri equation, which assumes that the density of Fat-Free Mass (muscle/bone) is exactly 1.100 g/cm³ for all humans. DEXA actually measures your specific bone mineral density, eliminating this assumption. Therefore, for extreme outliers (like heavily muscled athletes with very dense bones), DEXA is far superior to the BodPod.