Lean Body Mass Calculator
Estimate your lean body mass using three published equations — Boer, James and Hume — and see the implied body fat percentage.
Average lean body mass (kg)
57.2
- Boer (1984) LBM (kg)
- 58.1
- James (1976) LBM (kg)
- 59
- Hume (1966) LBM (kg)
- 54.4
- Implied body fat (% of weight)
- 23.7
Three published predictive equations estimate lean body mass from sex, weight and height. The average across Boer, James and Hume is a reasonable single figure; the implied body-fat percentage is the remainder of total weight. Predictive estimate for healthy adults only — not validated for children, pregnancy, or extreme body composition. For clinical decisions, use a DEXA scan or air-displacement plethysmography.
How to use this calculator
Pick your sex, then enter weight in kilograms and height in centimetres. The calculator returns the lean body mass predicted by the Boer (1984), James (1976) and Hume (1966) equations, the average of the three, and the implied body fat percentage (the share of weight that is not lean mass).
How the calculation works
Lean body mass (LBM) is everything in your body except stored fat — muscle, bone, organs, connective tissue and water. The Boer formula uses LBM = 0.407·W + 0.267·H − 19.2 for males and LBM = 0.252·W + 0.473·H − 48.3 for females, with W in kilograms and H in centimetres. The James formula uses LBM = 1.1·W − 128·(W/H)² (male) or LBM = 1.07·W − 148·(W/H)² (female). The Hume formula uses LBM = 0.32810·W + 0.33929·H − 29.5336 (male) or LBM = 0.29569·W + 0.41813·H − 43.2933 (female). Implied body fat % is (weight − LBM) ÷ weight × 100.
Worked example
A 70 kg, 180 cm male. Boer: 0.407·70 + 0.267·180 − 19.2 = 28.49 + 48.06 − 19.2 = 57.4 kg. James: 1.1·70 − 128·(70/180)² = 77 − 19.36 = 57.6 kg. Hume: 0.3281·70 + 0.33929·180 − 29.5336 = 22.97 + 61.07 − 29.53 = 54.5 kg. Average ≈ 56.5 kg, leaving about 13.5 kg of fat — an implied body fat of around 19%.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is your total weight minus stored fat. It includes muscle, bone, organs, blood, connective tissue and the water inside all of them. Some texts use "fat-free mass" interchangeably, though strictly speaking fat-free mass excludes essential lipids in cell membranes and the central nervous system while LBM includes them — the numerical difference is small.
Why do Boer, James and Hume give different numbers?
Each equation was fitted to a different reference population using a different gold-standard measurement (cadaver dissection for Hume, isotope dilution for Boer, anthropometry for James). For an average adult the three usually agree within a couple of kilograms. If they disagree by more, you sit outside the populations the equations were trained on — typically very lean, very muscular, very tall or very obese individuals.
How accurate is this versus a DEXA scan?
Predictive equations are quick and free but they only use sex, height and weight. A DEXA scan, hydrostatic weighing or air-displacement plethysmography directly measure body composition and are far more accurate, typically within 1–2% body fat. Use this calculator for a rough number; use a clinical measurement for anything that matters medically or for serious athletic preparation.
Should I use this for weight loss targeting?
It is a useful starting point. Daily protein targets are often expressed in grams per kg of LBM, and a sensible weight-loss goal preserves LBM while reducing fat mass. The number this calculator gives you is an estimate — track changes over weeks rather than treating any single reading as exact.
Is this calculator valid for children or pregnant women?
No. All three equations were derived from non-pregnant adults. Children, adolescents, pregnant women and people with substantial fluid imbalances (kidney disease, severe dehydration) are outside the validated range. For those populations, use a clinician-supervised measurement.
Does the implied body fat percentage match what scales show?
Not always. Consumer bioelectrical-impedance scales measure body composition via a tiny electrical current and apply their own formulas, often with poorly disclosed assumptions. The implied body fat % here is just (weight − estimated LBM) ÷ weight, derived from your reported height and weight. Differences of 3–6 percentage points between this calculator and a smart scale are normal.