Estimate your lean body mass using the Boer, James, and Hume formulas — see your fat-free tissue weight, body fat percentage, and set precision protein targets based on lean mass.
This lean body mass calculator uses two established formulas — Boer (1984) and James (the original Lean Body Mass equation) — to estimate the weight of your fat-free tissue: skeletal muscle, bone, organs, skin, and the water bound within those tissues. Enter your height, weight, and sex, and the calculator returns your estimated LBM in both pounds and kilograms, plus your estimated body fat weight (total weight minus LBM) and approximate body fat percentage. LBM is distinct from body mass index: BMI just divides total weight by height squared, while LBM specifically isolates the metabolically active, non-fat tissue. This number is particularly useful for setting protein targets based on lean mass rather than total body weight (which reduces the gram target for overweight individuals), for monitoring muscle gain or loss during body recomposition, and for clinical pharmacokinetic applications where lean tissue volume determines drug distribution. Results are estimates; clinical-grade measurement requires DEXA, hydrostatic weighing, or air displacement plethysmography.
Select your unit system, enter your height and weight, choose your sex, and calculate. The result shows LBM estimates from both the Boer and James formulas, an average of the two, estimated fat mass, and approximate body fat percentage. A result of "LBM: 138 lb (Boer) / 141 lb (James), Fat Mass: 37 lb, Body Fat: 21%" means roughly 138–141 lb of your body weight is lean tissue — muscle, bone, and organs. The body fat percentage is derived by dividing fat mass by total weight. If the two formula results are close (within 2–3 lb), you have high confidence in the estimate; wider divergence suggests you may fall outside the typical body proportions the formulas were calibrated on.
Example — 5'10" (177.8 cm), 185 lb (84 kg) man:
Boer: 0.407 × 84 + 0.267 × 177.8 − 19.2 = 34.2 + 47.5 − 19.2 = 62.5 kg (138 lb)
James: 1.1 × 84 − 128 × (84/177.8)² = 92.4 − 128 × 0.223 = 92.4 − 28.6 = 63.8 kg (141 lb)
Where W = body weight in kg, H = body height in cm.
The author suggests that this formula is applicable for children aged 13-14 years old or younger. The formula is used to compute an eLBM based on an estimated extracellular volume (eECV) as follows:
eECV = 0.0215 · W0.6469 · H0.7236
eLBM = 3.8 · eECV
In the formulas above, W is the body weight in kilogram and H is the body height in centimeter.
The terms are often confused but they're measuring opposite sides of the same coin. Lean body mass (LBM) is everything that isn't stored fat — muscle fibers, bone mineral, organ tissue, skin, and intracellular water. Body fat is the stored lipid tissue. Their sum equals total body weight. Body fat percentage expresses fat as a proportion of total weight; LBM percentage (sometimes called "lean percentage") is the remaining fraction. Importantly, LBM isn't synonymous with muscle mass — muscle is typically 40–50% of LBM in a well-trained adult, with the rest being bone (15%), organ tissue (25%), and other lean components (10–15%). If you're tracking a resistance training program, LBM changes are a better indicator of true muscle gain than weight alone because they factor out fat and water fluctuations. A person who gains 3 lb total but loses 2 lb of fat has actually gained 5 lb of lean mass — the scale doesn't show that; LBM tracking does.
No — and this matters if you're using LBM to set performance or nutrition targets. Muscle mass is only the skeletal muscle component of LBM; bone, organs, and structural water make up the rest. Most LBM formulas (including Boer and James) estimate total fat-free mass, not muscle mass specifically. Skeletal muscle mass is a narrower measure usually calculated via DEXA scan or by applying validated regression equations to LBM estimates. For most practical purposes — protein targets, metabolic rate estimates, fitness tracking — LBM is sufficient. But if you're a competitive bodybuilder, athlete, or patient with sarcopenia, a muscle mass measurement is more informative. Some research uses "fat-free mass" and "lean body mass" interchangeably; technically, true LBM includes essential fat (about 3% in men, 12% in women), while fat-free mass subtracts all fat including essential stores.
Lean body mass and fat free mass are often used interchangeably. While this is unlikely to cause issues in most cases, the two are not exactly the same.
Lean body mass includes the combined mass of bones, muscles, water, ligaments, tendons, and internal organs. Internal organs include some essential fat and the mass of this fat is included within the measurement of lean body mass. Although internal organs also have surrounding subcutaneous fat, this fat is not included within the measurement of lean body mass.
Fat free mass is calculated as the difference between total body mass and all fat mass including essential fat. This is the difference between fat free mass and lean body mass. Subtracting the mass of essential fat from lean body mass yields fat free mass. The difference between lean body mass and fat free mass amounts to approximately a 2-3% difference in men and 5-12% difference in women.
One of the most practical applications of LBM is protein target calibration. Most protein recommendations based on total body weight overestimate protein needs for people carrying significant fat mass, because fat tissue doesn't require protein for maintenance — only lean tissue does. Using LBM instead of total weight brings protein targets closer to actual biological need. The calculation is the same: multiply LBM (in kg) by the appropriate g/kg factor (1.2–2.0 for active adults). For a 240 lb (109 kg) moderately active woman with an estimated LBM of 127 lb (58 kg), the LBM-based protein target at 1.6 g/kg is 93g/day — substantially lower than the 174g/day the total body weight formula would give, and far more realistic as a sustainable daily intake. Lean athletes and people at healthy body fat percentages will find LBM-based and weight-based targets converge; it's primarily at higher body fat levels that the distinction matters.
Adding lean mass requires two simultaneous inputs: a mechanical stimulus (progressive resistance training) and adequate nutritional support (sufficient protein and total calories). Resistance training — specifically compound movements performed progressively heavier over time — triggers muscle protein synthesis via mechanical tension and metabolic stress pathways. Without a protein intake of at least 1.6 g/kg of LBM and sufficient total calories to sustain the repair process, training stimulus alone won't produce measurable lean mass gains. Two other factors are underappreciated: sleep and hormonal environment. Growth hormone and testosterone — both critical for muscle protein synthesis — are secreted primarily during deep sleep. Adults getting under 7 hours of sleep consistently show blunted lean mass gains from equivalent training compared to those getting 7–9 hours. A lean mass program that neglects sleep, protein timing, and progressive overload in favor of just "going to the gym" will underperform.
LBM peaks in most people during their late 20s to early 30s, then declines gradually — a process driven by declining anabolic hormones (testosterone, growth hormone, estrogen) and sedentary lifestyle compounding. Men typically carry higher absolute LBM than women at every age because of testosterone-driven muscle hypertrophy. Average LBM for a sedentary 5'10" male in his 30s is approximately 60–65 kg; for a sedentary 5'5" female, approximately 40–45 kg. After age 40, adults lose approximately 1–2% of lean mass per year without deliberate resistance training — the trajectory that leads to sarcopenia in later decades. Monitoring LBM over years, not just BMI, gives a far more sensitive signal of musculoskeletal health. The NIH identifies sarcopenia as a growing clinical concern in aging Americans, with implications for fall risk, metabolic health, and quality of life.
Height and weight are the two formula inputs — taller individuals of the same weight have lower estimated body fat and therefore higher estimated LBM, because height factors into lean tissue distribution. Sex shifts the formula constants substantially: women have proportionally less muscle and more essential fat than men at equivalent heights and weights. Current total body weight anchors the calculation, so LBM estimates are only as good as accurate weight measurement. Body proportions — how your muscle and fat are distributed — can push real LBM outside what formulas predict, particularly for people with atypical builds (very short torso, long limbs, or extremes of muscularity). The choice of Boer vs. James formula produces modestly different results; the average of both is the most defensible estimate.
Naomi is a 27-year-old in Portland, Oregon, 5'7" (170.2 cm), 138 lb (62.6 kg). Boer: 0.252 × 62.6 + 0.473 × 170.2 − 48.3 = 15.8 + 80.5 − 48.3 = 48.0 kg (106 lb). James: 1.07 × 62.6 − 148 × (62.6/170.2)² = 67.0 − 148 × 0.135 = 67.0 − 20.0 = 47.0 kg (104 lb). Average LBM ≈ 47.5 kg (105 lb), fat mass ≈ 15.1 kg (33 lb), body fat ≈ 24%. Her LBM-based protein target at 1.5 g/kg = 71g/day — much lower than the 94g/day her total weight would suggest, and easy to hit on a whole-food diet.
Elijah is a 33-year-old in San Antonio, 5'11" (180.3 cm), 220 lb (99.8 kg). Boer: 0.407 × 99.8 + 0.267 × 180.3 − 19.2 = 40.6 + 48.1 − 19.2 = 69.5 kg (153 lb). James: 1.1 × 99.8 − 128 × (99.8/180.3)² = 109.8 − 128 × 0.306 = 109.8 − 39.2 = 70.6 kg (156 lb). Average LBM ≈ 70 kg (154 lb), body fat ≈ 30%. His high absolute LBM reflects significant muscle mass but also a notable fat component. LBM-based protein target at 2.0 g/kg = 140g — versus 200g if based on total weight, a meaningful practical difference.
Recalculate LBM monthly during active body recomposition — as fat decreases and muscle increases, both the formula inputs and your LBM estimate change.
Use LBM rather than total body weight to set protein targets if you're at or above 25% body fat; it prevents over-estimating protein needs and makes daily targets more achievable.
Focus on compound resistance exercises (squat, deadlift, bench, row) as the most efficient stimulus for LBM gains rather than isolation work.
Don't confuse LBM loss with fat loss — rapid calorie restriction without adequate protein can cause significant LBM loss, which lowers TDEE and makes future fat loss harder.
For clinical or medical uses of LBM (e.g., discussing drug dosing with a pharmacist), use the average of Boer and James formulas as your reported estimate.
Track body weight and waist circumference alongside LBM estimates for a three-dimensional picture of body composition changes.
Lean body mass is the total weight of all body tissue that isn't stored fat — it includes skeletal muscle, bone, organs, skin, and the water within those tissues. It's everything you'd weigh if you could magically remove all your stored fat while keeping everything else.
The Boer formula for men: LBM (kg) = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2. For women: LBM (kg) = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3. The calculator above runs both Boer and James formulas and averages them.
No. Lean body mass includes muscle, bone, organs, and structural water. Muscle (skeletal muscle specifically) is typically 40–50% of total LBM. Fat-free mass and LBM are used interchangeably in most clinical contexts but are technically distinct.
Ranges vary significantly by height, sex, and training status. A sedentary adult man at 5'10" might have LBM of 60–65 kg; a trained man of the same height, 70–80 kg. Women's LBM is typically 10–20 kg lower at comparable heights due to hormonal differences in muscle mass.
Protein is needed to maintain and build lean tissue, not fat tissue. Basing protein targets on LBM rather than total body weight gives a more accurate picture of actual tissue need, especially for people carrying significant excess fat.
Progressive resistance training combined with adequate protein intake (1.6–2.0 g/kg LBM) and sufficient sleep is the evidence-based approach. Compound movements, consistent progressive overload, and 7–9 hours of sleep create the hormonal and nutritional environment for LBM gains.
Yes. Adults typically lose 1–2% of lean mass per year after 40 without deliberate resistance training — a process called sarcopenia. Regular strength training, adequate protein, and monitoring LBM over time are the best defenses against age-related muscle loss.
The Boer (1984) formula: Men: LBM (kg) = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2; Women: LBM (kg) = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3. It was derived from clinical populations and is one of the most cited LBM formulas in the literature.
Brief disclaimer: This calculator provides educational LBM estimates using the Boer, James, and Hume formulas. Results are estimates based on height, weight, and sex — clinical-grade measurement requires DEXA, hydrostatic weighing, or air displacement plethysmography. LBM estimates are useful for nutrition planning and fitness tracking but should not replace professional body composition assessment for medical purposes. Consult a healthcare provider or registered dietitian for personalized guidance.