Body Fat and Lean Mass Methodology

Inputs and outputs

Formula

fallback_body_fat = age_gender_average_table(age, gender)
visual_body_fat = male ? 5 + slider_pct * (40 - 5) : 12 + slider_pct * (50 - 12)
skinfold_sum = biceps + triceps + subscapular + suprailiac
L = log10(skinfold_sum)
body_density = Durnin_Womersley_coefficient(age, gender, L)
skinfold_body_fat = (495 / body_density) - 450
resolved_body_fat = clamp(chosen_or_fallback_body_fat, 3, 60)
lean_mass_kg = weight_kg * (1 - resolved_body_fat / 100)
fat_mass_kg = weight_kg - lean_mass_kg
body_surface_area = 0.007184 * height_cm^0.725 * weight_kg^0.425

Calculation steps

1. Use the user-provided or scan-estimated body fat percentage when present.
2. If no body-fat estimate is available, use the app demographic fallback table by age and gender.
3. For manual caliper mode, use the four-site Durnin-Womersley skinfold density table and convert density to body-fat percentage with the Siri equation.
4. Clamp impossible values, then compute lean mass and fat mass from body weight.
5. Compute body surface area with the Du Bois formula for context.

Guardrails

• Body-fat estimates are planning estimates, not a diagnostic body-composition test.
• The demographic fallback is intentionally treated as lower confidence than a direct measurement.
• The visual slider is a rough estimate and should not be treated as a medical measurement.

Sources

• Body fat assessed from total body density and its estimation from skinfold thickness British Journal of Nutrition / PubMed
• Simple measures: skinfolds Measurement Toolkit
• Body Composition, Morbidity, and Mortality NCBI Bookshelf
• Body Surface Area NCBI Bookshelf