Hydrostatic Weighing & Bod Pod (Densitometry)

Densitometry estimates percent body fat from whole-body density (Db = mass ÷ volume), exploiting that fat is less dense (~0.90 g/mL) than fat-free mass (~1.10 g/mL).

Hydrostatic (underwater) weighing — the classic laboratory standard. The person is weighed on land and again fully submerged after a maximal exhalation; the weight difference gives body volume (Archimedes' principle), corrected for residual lung volume (a key error source if estimated rather than measured).

Air-displacement plethysmography (Bod Pod) — the person sits in a sealed chamber that measures air displacement to get body volume; quick, dry, and comfortable, with accuracy comparable to hydrostatic weighing and similar reliance on lung-volume correction.

Convert density to %fat:

• Siri (1961): %fat = (495 / Db) − 450
• Brozek (1963): %fat = (457 / Db) − 414.2

Accuracy: criterion-level (~±2–3%) when residual volume is measured and protocols are followed; assumes a fixed fat-free-mass density that can be off in athletes, older adults, and some ethnic groups. Bod Pod's convenience makes it common in sports/clinical settings.

• Estimates %fat from whole-body density (fat ~0.90 vs. fat-free ~1.10 g/mL).
• Hydrostatic = water displacement (correct for residual lung volume); Bod Pod = air displacement.
• Density → %fat via Siri (495/Db − 450) or Brozek (457/Db − 414.2).
• Criterion-level accuracy (~±2–3%); residual-volume error and assumed FFM density are limits.