# How Does Hydrostatic Weighing Help in the Estimation of Body Fat?

Hydrostatic weighing for body fat estimation is a means of analyzing the fitness of a person. We shall understand its principle of working, and highlight the steps used in calculating an individual's body fat percentage, through it.

Satyajeet Vispute

Last Updated: Jan 17, 2019

Fun Fact

It is believed that Archimedes came up with his principle by observing water being displaced as he got into his bathtub. The story goes, that he got so excited by his discovery, that he forgot to dress, and ran stark naked through the streets, shouting "Eureka"!

Working Principle

To understand the working principle of hydrostatic weighing, we first need to learn about buoyancy, or the ability of an object to float on a liquid.

How Does Hydrostatic Weighing Work

Hydrostatic weighing is also known as underwater weighing. The following is its two-step procedure.

**Step 1**:

**Calculation of Body Density**

To calculate the body density of a person, a few measurements have to be taken. First, the person is weighed on dry ground. This weight is noted down as the person's Dry Weight.

**Body Density = Dry Weight ÷ [((Dry Weight - Wet Weight) ÷ Water Density) - RV - 0.1]**

**Step 2: Calculation of Body Fat**

To calculate body fat using the body density measured in the previous step, either the Brozek formula or the Siri formula is used.

*Siri Formula:*

**Percentage Body Fat = [{4.95 ÷ Body Density} - 4.50] × 100**

*Brozek Formula:*

**Percentage Body Fat = [{4.570 ÷ Body Density} - 4.142] × 100**

Limitations of Hydrostatic Weighing

1) The biggest source of error in hydrostatic weighing is the calculation of RV. This RV measures the volume of air remaining in the lungs after complete exhalation.