Fitness Tools | Calculating Body Composition

This module is a collection of the most popular body fat percentage equations, calculated by measuring various skinfold sites in millimeters.

Workflow

The typical workflow for calculating body fat from skinfolds:

1. Collect measurements from the skinfold sites required by your chosen equation.
2. Calculate body density.
3. Convert body density to body fat using one of the provided equations.

Density-to-Body-Fat Conversions

Every subclass in this module inherits from GenericCalculator, which exposes five methods that convert body density (g/cm³) to body fat percentage. These are not interchangeable alternatives — each was validated on a specific population, and picking the wrong one introduces systematic bias.

Method	Intended population	Formula
siri()	General — the standard default	%BF = 495/ρ − 450
brozek()	General — alternative default with refined fat-free-mass assumptions	%BF = 457/ρ − 414.2
schutte()	African-American men	%BF = 437.4/ρ − 392.8
wagner()	Black male athletes	%BF = 486/ρ − 439
ortiz()	Black women	%BF = 483.2/ρ − 436.9

These conversions are required for every equation in this module except JacksonPollock4Site, which computes body fat directly from skinfolds via its own body_fat() method.

If your subject doesn’t fit one of the specialized populations, use siri() — it is the broadly taught default (ACSM, Heyward). Use brozek() when a refined fat-free-mass model is preferred. Only use schutte(), wagner(), or ortiz() when the subject matches the target population for that equation.

Constructor Arguments

All skinfold classes take the same three positional arguments:

1. Age — positive integer
2. Sex — "male" / "female" string, or the typed Sex.MALE / Sex.FEMALE enum
3. Skinfold measurements — tuple of integers in millimeters. Order does not matter.

The Sex enum is a StrEnum, so Sex.MALE == "male" — you can mix either form.

Example: Durnin–Womersley (4-site)

A 40-year-old female with skinfolds:

• triceps = 7 mm
• biceps = 5 mm
• subscapular = 4 mm
• suprailiac = 10 mm

>>> from fitness_tools.composition.bodyfat import DurninWomersley
>>> from fitness_tools.types import Sex
>>> calc = DurninWomersley(40, Sex.FEMALE, (7, 5, 4, 10))
>>> calc.body_density()
1.046703631104186

>>> calc.siri(calc.body_density())
22.9

By the Durnin–Womersley equation, our hypothetical subject’s body fat is 22.9%.

Example: Jackson–Pollock 4-site (direct body fat)

As noted above, JacksonPollock4Site is the one equation in the module that converts skinfolds directly into body fat without an intermediate density-to-BF step.

A 25-year-old male with skinfolds:

• abdominal = 6 mm
• triceps = 5 mm
• thigh = 8 mm
• suprailiac = 6 mm

>>> from fitness_tools.composition.bodyfat import JacksonPollock4Site
>>> from fitness_tools.types import Sex
>>> calc = JacksonPollock4Site(25, Sex.MALE, (6, 5, 8, 6))
>>> calc.body_fat()
5.2

Our hypothetical subject has a body fat of 5.2%.

Available Equations

Class	Sites	Conversion
DurninWomersley	4 (triceps, biceps, subscapular, suprailiac)	density → siri() etc.
JacksonPollock7Site	7	density → siri() etc.
JacksonPollock4Site	4 (abdominal, triceps, thigh, suprailiac)	direct body fat via body_fat()
JacksonPollock3Site	3 (sex-specific sites)	density → siri() etc.

For the underlying formulas, age brackets, and sex-specific coefficients see the Body Composition skill reference.