What is Hydrostatic Weighing and how does it measure body composition?

Overview

Hydrostatic weighing — also called hydrodensitometry or underwater weighing — is a method of assessing body composition by measuring how much water a person displaces when fully submerged. It was previously considered the gold standard for body composition measurement and is still widely used at the research level.

Important: Hydrostatic weighing is an indirect measurement. Body composition is not read directly; instead, body density is calculated first, and body composition is derived from that value.

How It Works

The method is based on Archimedes' principle: when a body is fully submerged in water, the volume of fluid displaced equals the volume of the body.

The formula:

Body Density = Mass ÷ Volume

During the assessment:

1. The client enters a water tank and sits on a submerged scale.
2. The client exhales as much air as possible.
3. Their weight underwater is recorded.
4. The displacement of the water is measured to calculate body volume.
5. Body density is calculated and used to estimate body composition.

Key physiological principle: Fat mass is less dense than fat-free mass. As a result, individuals with a higher percentage of body fat will weigh less underwater.

Notes

• Strict pre-assessment protocols must be followed. Eating, drinking, and exercising before the test can affect measurement reliability.
• Use the same procedures every time you assess a client to ensure consistent, comparable results.
• Due to cost and space requirements, this method is typically used in research or clinical settings rather than general fitness facilities.

If this resolves your issue, no further action is needed.

If the problem persists, contact support and include: the assessment protocol used and any pre-assessment conditions that may have varied.

Applies to: Styku body composition education resources; Health Tech Assessment curriculum, Lesson 9