How the BMI body fat estimator works
What Deurenberg does
The 1991 formula converts BMI, age, and sex into an estimated body fat percentage using population regressions. It is handy precisely once — as a no-tape ballpark — and weak everywhere else, because it cannot see the two things that dominate individual composition: muscle mass and fat distribution.
Where it breaks
Athletes read 3–8 points high (BMI counts their muscle as if it were fat). Older adults read somewhat better (muscle loss converges them toward population averages). Very lean individuals read low. The error bands — ±4–6% — mean a 22% estimate could be an 18 or a 26 in reality. Treat it as climate, not weather.
The hierarchy of estimates
DEXA/hydrostatic (lab, ±1–2%) → Navy tape (±3–4%, repeatable) → impedance scales (±5–8%, hydration-sensitive) → Deurenberg (±4–6%, no measurement needed). Each tier buys accuracy with effort. For tracking change — the thing that matters — consistency beats accuracy, which is why the tape method wins in practice.