Patient-Friendly Explanation

Hydrostatic weighing, also known as underwater weighing, is one of the most accurate ways to measure your body fat percentage. You sit on a special chair that is lowered into a tank of water while you exhale all the air from your lungs. Because muscle sinks and fat floats, the difference between your weight on land and in water tells us exactly how much of your weight is fat versus lean body mass.

Clinical Definition

Hydrostatic weighing (densitometry) estimates body density by applying Archimedes’ principle: the buoyant force on a submerged body equals the weight of displaced water. Body density is calculated as mass-in-air divided by body volume (mass-in-air minus submerged mass, corrected for water density and residual lung volume). The Siri or Brozek equation then converts body density to body fat percentage. It has long been considered a gold-standard method with an error margin of approximately 1.5–2.5%.

Why It Matters

While less commonly used today due to the availability of DEXA and BIA, hydrostatic weighing remains a reference standard in body composition research. Understanding the principle behind it helps patients appreciate that weight on the scale alone does not distinguish between fat and lean tissue — a core message in NuLifeCare Singapore’s holistic weight management programme.