In more detail
Body fat percentage is fat mass divided by total weight. Laboratory methods such as DEXA scans and underwater weighing are the most accurate; everyday estimates come from bioimpedance scales, skinfold calipers or tape formulas. The US Navy formula for men, in centimetres, is 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450; the version for women adds the hip measurement. Lean body mass is weight minus fat mass, and FFMI adjusts it for height.
Why it matters
It separates fat from muscle in a way BMI (body mass index) cannot. Commonly cited reference ranges (American Council on Exercise) put average adults at about 18 to 24% for men and 25 to 31% for women, with 25% and 32% or more classed as obese; women carry more essential fat. Tape and scale estimates can be several percentage points off, and bioimpedance shifts with hydration, so track the trend with one method rather than comparing figures from different ones.
Example
Man, 180 cm, neck 38 cm, waist 85 cm, weight 80 kg (US Navy formula):
495 / (1.0324 − 0.19077 × log₁₀(47) + 0.15456 × log₁₀(180)) − 450 = 16.1 %
Fat mass = 80 × 0.161 = 12.9 kg
Lean mass = 80 − 12.9 = 67.1 kg