Lean Body Mass Calculator

What lean body mass actually is

Lean body mass (LBM) is your total body mass minus fat mass. It includes muscle, bone, organs, skin, connective tissue, and the small amount of essential fat embedded in those structures (cell membranes, brain, marrow). In men, essential fat is roughly 3% of body mass; in women it's ~12%, primarily in the breast and hip region required for normal hormonal function. Subtract everything except the storage adipose tissue you can change through diet and exercise, and you have LBM.

The three formulas — why they disagree

Boer (1984): derived from hydrodensitometry in 81 men and 73 women. Men: 0.407 × weight (kg) + 0.267 × height (cm) − 19.2. Women: 0.252 × weight + 0.473 × height − 48.3. The Boer formula has become the de facto standard in clinical pharmacology because it correlates closely with DEXA-measured lean mass and behaves well across a wide BMI range.

James (1976): men: 1.10 × weight − 128 × (weight² / height²). Women: 1.07 × weight − 148 × (weight² / height²). Derived from British adult body composition data using underwater weighing.

Hume (1966): men: 0.32810 × weight + 0.33929 × height − 29.5336. Women: 0.29569 × weight + 0.41813 × height − 43.2933. Based on potassium-40 counting. The oldest of the three; tends to over-estimate LBM at weights above 90 kg because the reference population was relatively lean.

Using LBM for protein targets

Per-body-weight protein targets break down at the extremes of body fat. A 220 lb person at 35% body fat doesn't need 200g of protein/day — they have far less metabolically active tissue than the per-weight math implies. The Helms 2014 review in JISSN recommended 2.3–3.1 g/kg of LBM for natural lifters in a caloric deficit, which gives the right target across all body compositions. For a 220 lb (100 kg) person at 35% body fat, LBM ≈ 65 kg, so protein target ≈ 150–200g — substantially less than the 230g a per-total-weight calculation would suggest.

Using LBM for BMR — the Katch-McArdle formula

The Mifflin-St Jeor BMR equation uses total body weight, so it slightly under-estimates BMR in very lean people and over-estimates in obese people. Katch-McArdle solves this by using LBM directly: BMR = 370 + (21.6 × LBM in kg). For most healthy adults, the two formulas converge within ±50 kcal/day. For competitive bodybuilders, ultra-endurance athletes, and people with severe obesity, Katch-McArdle is the more accurate choice — assuming you have a reliable LBM number.

Tracking LBM over time during a cut

The whole point of "preserving muscle during fat loss" is preserving LBM. A scale weight drop of 5 lbs from 200 to 195 is meaningless without knowing the split: -2 lbs fat / -3 lbs LBM is a bad outcome; -5 lbs fat / 0 LBM is a great outcome. The cheapest way to track this is monthly DEXA scans ($45–75 in most US cities) or biweekly bioelectrical impedance with a quality scale (less accurate, but consistent enough for trends). Aim for >80% of total loss coming from fat — preserve LBM with adequate protein (≥1.8 g/kg total body weight) and resistance training 3+ times/week.

Related tools

• Body Fat % Calculator (US Navy)
• BMR Calculator (incl. Katch-McArdle)
• Protein Calculator
• Ideal Weight Calculator

Frequently asked questions

What's the difference between lean body mass and fat-free mass?

Technically they're slightly different — lean body mass (LBM) includes a small amount of essential fat (~3% in men, ~12% in women), while fat-free mass (FFM) excludes all fat. In casual usage they're used interchangeably, and most formulas (Boer, James, Hume) produce LBM, not strict FFM. DEXA scans separate total mass into bone, lean tissue, and fat — DEXA "lean mass" maps closely to LBM. For drug dosing protocols using "ideal body weight" or LBM, the small distinction rarely matters.

Why do different LBM formulas give different answers?

Each formula was derived from a different reference population and method. Boer 1984 was derived from underwater weighing (hydrodensitometry) in 81 men and 73 women. James 1976 was based on body composition studies of British adults. Hume 1966 used potassium-40 counting, an older technique. Boer tends to track closer to modern DEXA measurements; Hume overestimates LBM at body weights above 90 kg. For most healthy adults, the three formulas land within 3–4 kg of each other — the calculator shows all three so you can see the spread.

How accurate are these formulas vs DEXA?

In healthy adults, mean error vs DEXA is ±2–4 kg, with individual error up to ±5–8 kg in outliers. Accuracy is worst at the extremes — very lean athletes (low body fat) and people with severe obesity. The Boer formula was validated against hydrodensitometry, not DEXA; modern DEXA correlates well but slightly underestimates LBM at high body fat percentages. For drug dosing purposes (gentamicin, vancomycin, chemotherapy), the formula error is small enough that clinical guidelines accept calculated LBM as a starting point.

What can I do with my LBM number?

Three practical uses. First, set a protein target — recommendations of 2.3–3.1 g/kg of LBM (Helms 2014, JISSN) for muscle preservation during a cut translate to a more accurate intake than per-total-body-weight targets, especially at higher body fat percentages. Second, compute BMR via Katch-McArdle: BMR = 370 + (21.6 × LBM in kg). This is more accurate than Mifflin-St Jeor for very lean or very heavy people. Third, track LBM change over time — losing LBM during a cut signals too aggressive a deficit or insufficient protein.

Can I increase my lean body mass?

Yes — through resistance training plus adequate protein and a small calorie surplus (or recomposition at maintenance for beginners). Trained, well-fed adults can add 0.5–1.0 kg of lean mass per month in the first year, dropping to 0.1–0.3 kg/month after several years of training. Beginners often see 1–2 kg in the first 3 months as muscle glycogen, water, and neural adaptations contribute alongside true hypertrophy. After age 40, anabolic resistance slightly reduces the rate, but the same training and protein strategies still work.