Why Lean Mass Matters More Than Weight
Lean body mass is everything except fat: muscle, bone, organs, connective tissue and body water. Two people at 80 kg with 12% and 30% body fat carry 70 kg and 56 kg of lean tissue respectively — 14 kg apart in the tissue that actually determines metabolic rate, strength and drug clearance.
This is why lean mass, not total weight, is the correct basis for protein targets, for calorie formulas like Katch-McArdle, and for dosing anaesthetics and chemotherapy.
Three Formulas
- Boer (1984) — the most widely used in clinical settings, validated across a broad population.
- James (1976) — older, and known to underestimate in obese individuals because of its squared weight-to-height term.
- Hume (1966) — the original equation, still used in some pharmacology references.
All three estimate from height and weight alone and cannot know your actual composition. Where a body fat measurement exists, using it directly is always more accurate than any formula.
Typical Body Fat Ranges
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Women carry more essential fat for hormonal and reproductive function — the difference is biological, not a matter of conditioning.
Fat-Free Mass Index
FFMI is lean mass divided by height in metres squared — BMI for lean tissue. It is more informative than BMI for anyone who trains. Untrained men sit around 18–19; well-trained natural lifters reach 22–23; values above about 25 are rarely achieved without drugs.
Measuring Body Composition Properly
DEXA is the practical gold standard, accurate within 1–2%. Hydrostatic weighing and the Bod Pod are comparable. Bioelectrical impedance scales are convenient and vary by several percentage points with hydration, meal timing and skin temperature — useful for tracking direction, not for a single reading. Skinfold calipers in skilled hands are more accurate than most people expect and far less so in unskilled ones.
Frequently Asked Questions
How much lean mass can I gain?
A trained beginner might add 0.5–1 kg a month in the first year, dropping to a few kilograms a year afterwards. Progress is not linear and slows sharply with training age.
Do I lose lean mass when dieting?
Some, unavoidably. Adequate protein — 1.8–2.4 g per kg — combined with resistance training minimises it. Aggressive deficits without training can lose 30–40% of the total as lean tissue.
Which formula should I trust?
Boer for most people. But if you have a recent DEXA or a reliable body fat figure, enter it — measurement beats estimation every time.