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Lean Body Mass Calculator

Estimate your lean body mass, body fat percentage, and muscle composition using multiple scientific formulas.

Boer · James · Hume formulas, side-by-sideLast updated 2026-05-15

Formulas typically agree within ±2–3 kg of DEXA for healthy adults — educational estimate only.

Kilograms and centimeters.

cm
kg

What this tool computes

Three peer-reviewed formulas — Boer, James and Hume — estimate your lean body mass (LBM) from height, weight and sex. Body fat percent and fat mass are derived from LBM. Boer is highlighted as the recommended pick for most adults.

Enter your height and weight to estimate your lean body mass and body composition.

What Is Lean Body Mass?

Lean body mass (LBM) is everything in your body that isn't fat — skeletal muscle, organs, bone, connective tissue, body water and a small amount of essential fat that the body cannot live without. It is also called fat-free mass, and the two terms are used interchangeably in most clinical literature.

LBM matters because it drives resting metabolic rate, strength output, and the way your body responds to nutrition. A higher LBM means you burn more calories at rest, recover faster from training, and tolerate larger meals without storing as much fat. It's also the figure clinicians use to dose certain medications.

Lean Mass vs Muscle Mass — They're Not the Same

People often use "lean mass" and "muscle mass" as if they mean the same thing. They don't. Lean mass is the larger umbrella — it contains muscle, plus bone, organs and water. Skeletal muscle mass is just one component, and it's typically 40–50% of total lean mass in an adult.

Lean Body Mass includes

  • • Skeletal muscle (the part you train)
  • • Smooth and cardiac muscle
  • • Bones, ligaments, tendons
  • • Organs (liver, kidneys, brain, etc.)
  • • Body water
  • • A small share of essential fat

Skeletal Muscle Mass is

  • • A subset of lean mass
  • • The tissue that responds to resistance training
  • • ~40–50% of total LBM in adults
  • • Measured separately by DEXA or MRI segmental analysis

Why Body Fat Matters Alongside LBM

Body fat percentage is the flip side of lean body mass — if you know one, you know the other. Tracking both is what separates a meaningful body composition assessment from a bathroom-scale number. Two people at the same weight and height can have wildly different fat-to-lean ratios, and that ratio is what actually drives strength, conditioning, longevity markers, and how clothes fit. A stable scale weight with rising lean mass and falling fat mass is one of the strongest signals that your training and nutrition are working.

Which Lean Body Mass Formula Is Most Accurate?

All three formulas in this calculator estimate LBM from height, weight and sex — but they were calibrated on different reference populations and against different criterion methods. That's why two formulas can disagree by 2–3 kg for the same person.

Boer (1984) · Recommended

Developed for clinical drug-dosing. Widely cited as the most accurate of the three for healthy adult men and women across a broad BMI range.

M: 0.407·W + 0.267·H − 19.2
F: 0.252·W + 0.473·H − 48.3

James (1976)

Older quadratic model. Tends to overestimate LBM in lean, taller individuals and underestimate in shorter, heavier ones — useful as a cross-check.

M: 1.1·W − 128·(W/H)²
F: 1.07·W − 148·(W/H)²

Hume (1971)

The earliest of the three. Slightly larger error band but still inside ±3 kg of DEXA for most adults. Often used alongside Boer in clinical work.

M: 0.3281·W + 0.33929·H − 29.5336
F: 0.29569·W + 0.41813·H − 43.2933

W = weight in kilograms · H = height in centimeters. US and stone inputs are converted to metric internally.

How To Increase Lean Body Mass

Train each muscle 2× per week

Hypertrophy research consistently shows a sweet spot of 10–20 hard sets per muscle group per week, split across two or three sessions.

Eat 1.6–2.2 g of protein per kg

Higher protein intake preserves lean mass during a cut and supports muscle protein synthesis during a building phase.

Stay in a small calorie surplus to add mass

About +200 to +400 kcal above maintenance is enough for most lifters. Larger surpluses just add more fat per kg of lean mass gained.

Sleep 7–9 hours

Under-sleep cuts testosterone, raises cortisol, and blunts the muscle-building response to training even when calories and protein are perfect.

Progressive overload

Lean mass adapts to the demand placed on it. Slowly add weight, reps, or quality sets across months — not just within a single session.

Re-measure every 4–6 weeks

Lean mass moves slowly — typically 0.25–0.5 kg per month even with great inputs. Don't re-test weekly; the noise will swamp the signal.

Disclaimer: This calculator provides estimated body composition values and should not replace professional medical or fitness assessment. Boer, James and Hume were validated on adult populations and carry roughly ±2–3 kg of error compared with DEXA. Consult a registered dietitian, physician, or exercise physiologist before making significant changes to your nutrition or training plan.

Frequently Asked Questions

Lean body mass (LBM) is the weight of everything in your body that isn't fat — muscle, bone, organs, body water and a small share of essential fat. It is often used interchangeably with the term 'fat-free mass'. LBM matters because it drives resting metabolic rate, strength capacity, and how the body responds to nutrition.

This calculator estimates LBM with three peer-reviewed formulas — Boer (1984), James (1976) and Hume (1971). Each uses height, weight and sex to predict LBM. Body fat percentage is then derived by subtracting LBM from total weight and expressing the difference as a share of body weight.

Each formula was calibrated on a different population sample using a different criterion method (densitometry, hydrostatic weighing, isotope dilution). That's why they can disagree by 2–3 kg for the same person. Treat the spread as a confidence band — Boer is typically the most accurate for healthy adults.

No. Skeletal muscle mass is one component of lean body mass — typically 40–50% of it. LBM also includes bone, organs, connective tissue and body water. When someone says they 'gained muscle', they usually mean they gained lean mass; only DEXA or MRI segmental analysis can isolate skeletal muscle specifically.

Yes — through progressive resistance training, adequate protein (1.6–2.2 g/kg of bodyweight), a small calorie surplus, and consistent sleep. Healthy adults typically add 0.25–0.5 kg of lean mass per month early on, slowing to 0.1–0.2 kg per month after the first year or two of serious training.

Boer is the default recommendation for most healthy adults because it has the smallest error band against DEXA in published comparisons. James and Hume are useful cross-checks — if all three agree, your estimate is reliable; if they disagree by more than 3 kg, your height or weight may be at the edge of the formulas' calibration range.

Height-and-weight regressions carry roughly ±2–3 kg of error compared with DEXA scan for healthy adults. They are less reliable at the extremes — very lean physique athletes, very tall or very short people, and clinically obese individuals — where direct measurement methods (DEXA, BodPod, hydrostatic weighing) are preferred.

In most modern usage they are treated as the same thing. Technically, 'fat-free mass' excludes 100% of body fat, while 'lean body mass' includes a tiny amount of essential fat embedded in cell membranes and organs. The numerical difference is small (around 2–3% of body weight) and most consumer-facing calculators, including this one, treat the two as equivalent.

No. Boer, James and Hume were developed and validated on adult populations. Paediatric body composition uses different reference equations and percentile charts, and decisions for children should always involve a paediatric professional. This calculator declines to return a value when you indicate age 14 or younger.

No. A DEXA or BodPod measurement is far more accurate than any height-and-weight regression and can break lean mass down regionally. Use this calculator as a quick screening tool and a way to track relative change over months. If you need a precise number — for medication dosing, a physique contest, or a clinical study — book a direct measurement.

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