Body Fat Calculator (Siri Equation)

How to calculate body fat from density?

The Siri equation is a widely used densitometric model that converts whole-body density into a percentage of fat mass.

Formula: BF% = ((4.95 / Density) - 4.50) × 100

Using the Siri calculator: an example

Suppose a person has a measured body density of 1.050 g/cm³ from hydrostatic weighing.

Step-by-step calculation:

• Divide 4.95 by the density (4.95 / 1.050 = 4.714)
• Subtract 4.50 from the result (4.714 - 4.50 = 0.214)
• Multiply by 100 to get the percentage (0.214 × 100 = 21.4%)

Frequently Asked Questions

What is the Siri Equation?

It is a formula developed by William Siri in 1956 to estimate body composition based on the different densities of fat and fat-free mass.

Is it more accurate than BMI?

Yes, because it relies on actual body density measurements rather than just height and weight ratios.

When should I use this?

Use this when you have density data from methods like underwater weighing or air displacement plethysmography (Bod Pod).