Get lean body mass from weight and body fat %, or estimate LBM with Boer, James, and Hume when you do not have a body fat reading—all in your browser, nothing uploaded.
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Lean body mass (LBM) is total body weight minus fat mass. Calculate it from body fat % directly, or estimate using height/weight formulas (Boer, James, Hume) when body fat % is unavailable.
LBM estimates use validated formulas (Boer, James, Hume) but are approximations with ±2–4 kg error margin. For clinical body composition assessment, use DEXA scan or hydrostatic weighing. Results are for fitness planning only.
Lean body mass (LBM) — also called fat-free mass (FFM) — is everything in your body except fat: muscles, bones, organs, water, and connective tissue. It is important for dosing calculations, setting protein targets, and tracking body composition changes.
**Method 1 — From body fat %:**
LBM = Total weight × (1 − Body fat % / 100)
Example: 80 kg person with 20% body fat: LBM = 80 × 0.80 = **64 kg**
**Method 2 — Estimation formulas (when body fat % is unknown):**
• **Boer formula** (most accurate for general population):
Male: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Female: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
• **James formula** (commonly used in clinical settings):
Male: LBM = 1.1 × weight(kg) − 128 × (weight/height in cm)²
Female: LBM = 1.07 × weight(kg) − 148 × (weight/height in cm)²
• **Hume formula** (uses both weight and height):
Male: LBM = 0.3281 × weight(kg) + 0.3975 × height(cm) − 29.5336
Female: LBM = 0.29569 × weight(kg) + 0.41813 × height(cm) − 43.2933
All formula methods carry ±2–4 kg error compared to DEXA — use the body-fat-% method if you have an accurate body fat measurement.
If you know your body fat percentage (from DEXA, navy method, or calipers), use 'From Body Fat %' for the most accurate result. If not, use one of the formula methods (Boer recommended).
Enter weight in kg or lbs and height in cm or feet/inches. The calculator converts units automatically.
Use your most recent body fat measurement. US Navy circumference method results, DEXA scan, or bioelectrical impedance readings all work here.
The result shows LBM, fat mass, and (if applicable) recommended protein intake based on lean mass.
Common real-world scenarios where this tool saves time.
Protein recommendations are more accurate per kg of lean body mass than per kg of total body weight. A 100 kg person with 35% body fat has only 65 kg LBM — protein target of 1.6g/kg LBM = 104g vs 160g/total weight. Using LBM prevents overestimates for higher body fat individuals.
When cutting (losing fat while maintaining muscle), track LBM over time. Stable or increasing LBM while weight drops confirms you are losing fat, not muscle.
The Katch-McArdle BMR formula uses LBM directly: BMR = 370 + (21.6 × LBM in kg). More accurate than Mifflin-St Jeor for athletes and higher body fat individuals.
Many medications (especially IV drugs) are dosed per kg of lean body mass or ideal body weight rather than total weight. The Boer and Hume formulas are used in clinical pharmacokinetics.
Step-by-step chains that connect related tools for common tasks.
Use direct LBM for Katch-McArdle BMR and g/kg protein targets during a cut or bulk.
Accuracy and accessibility of different lean body mass measurement approaches.
| Method | Accuracy vs DEXA | Equipment needed | Cost |
|---|---|---|---|
| Body fat % + weight | ±1–3% | Body fat measurement | Free–$300 |
| DEXA scan | Gold standard | DEXA machine | $50–$200/scan |
| Boer/James/Hume formulas | ±2–4 kg | Scale + tape measure | Free |
| Hydrostatic weighing | ±2–3% | Specialized tank | $50–$100 |
| Bioelectrical impedance | ±3–8% | BIA scale or device | $30–$200 |
For most fitness tracking, the Boer formula or body fat % method is accurate enough. DEXA is only necessary for clinical or research purposes.
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In practice, the terms are used interchangeably. Technically, lean body mass includes essential fat (lipids in cell membranes and organs — about 3% in men, 12% in women), while fat-free mass (FFM) excludes all fat. The difference is small for most practical purposes.
The Boer formula generally shows the smallest average error across studies for mixed populations. The James formula is widely used in clinical pharmacology. All three (Boer, James, Hume) have similar accuracy (±2–4 kg compared to DEXA). Using your body fat % directly (if measured accurately) is more accurate than any formula.
LBM as a percentage of body weight = 100% − body fat%. Typical ranges: Men: 80–90% LBM (10–20% body fat). Women: 68–84% LBM (16–32% body fat). Athletes may have 85–95% LBM. The LBM percentage itself is not typically a direct health metric — body fat % is the more useful benchmark.
The primary driver is resistance training (progressive overload) combined with adequate protein (1.6–2.2g/kg of body weight or LBM) and a slight caloric surplus (100–300 kcal/day above maintenance for muscle gain). Sleep (7–9 hours) and recovery are also essential — most muscle protein synthesis occurs outside the gym.
No — LBM includes all non-fat tissue: skeletal muscle (~40–45% of body weight), bones (~15%), organs (~15%), blood (~7%), skin (~5%), and water. Muscle mass specifically refers to skeletal muscle. When people say 'build lean mass', they typically mean muscle, but the calculator reports total non-fat mass.
Yes — this is called 'recomposition' (body recomposition) and is the goal of most fitness programs. A moderate calorie deficit (−300–500 kcal), high protein intake (1.6–2.2 g/kg), and resistance training can preserve or even increase LBM while reducing fat mass. Progress is slower than aggressive cuts but more sustainable.
DEXA (dual-energy X-ray absorptiometry) is the gold standard for body composition, accurate to within 1–2% for fat mass and LBM. It measures bone density, fat mass, and lean soft tissue separately. Other methods: hydrostatic weighing (2–3% error), air displacement plethysmography (2–3%), bioimpedance (3–8%), calipers (3–5% with skilled technician).
Height, weight, and body-composition inputs are calculated locally in your browser—they are not uploaded to EverydayTools servers.
Formula accuracy ±2–4 kg vs DEXA. For clinical drug dosing or medical decisions, use a measured body composition assessment. Health information is for educational purposes only and is not medical advice. Consult a qualified healthcare provider before making health decisions.
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Reviewed on 2026-06-08.
Calculated in your browser — not uploaded. Share a link to restore these inputs.
Use this if you have a body fat % from a scale, calipers, or DEXA—results are exact for that input. Keys: 1 direct · 2 estimate
From scale, calipers, or DEXA (0–100%).
56 kg
123.5 lb
About 80% of your body is lean mass. Fat mass: 14 kg.
BMR is estimated from your lean mass; use it as a base for daily calories.
Protein
Daily protein (by LBM): about 90–123 g for active adults. Guidance only.
BMR
Resting calories per day. Multiply by an activity factor to estimate total daily needs.
Target weight at a body fat %
Assumes your lean mass stays constant.
≈ 65.9 kg at 15% body fat