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Lean Body Mass Calculator

Calculate lean body mass and fat mass from height and weight using the Boer, James and Hume formulas, or directly from a known body fat percentage.

61.4 kg
Lean body mass (Boer (1984))
18.6 kg
Fat mass
19.0 kg/m²
Lean mass index

Every formula, side by side

MethodLean massImplied body fatFormula
Boer (1984)61.4 kg23.2%0.407 × kg + 0.267 × cm − 19.2
James (1976)62.7 kg21.6%1.1 × kg − 128 × (kg ÷ cm)²
Hume (1966)57.8 kg27.8%0.32810 × kg + 0.33929 × cm − 29.5336

Boer, James and Hume were each regression-fitted to different reference populations, so they routinely disagree by several kilograms for the same person — that spread is a fair signal of their real error. They assume average composition for your size, so a bodybuilder and a sedentary person of identical height and weight get the same answer. Estimates only, not medical advice.

What is the Lean Body Mass Calculator?

Lean body mass is your total weight minus your fat mass — muscle, bone, organs and water. It can be estimated from height and weight using the Boer, James or Hume formulas, or calculated exactly from a measured body fat percentage.

  • Boer, James and Hume formulas computed together
  • Direct lean mass from a known body-fat percentage
  • Reports fat mass and lean body mass index
  • Metric and imperial input on one toggle
  • Formulas printed alongside the result so they are auditable
  • Runs offline in your browser — nothing is uploaded

How to use the Lean Body Mass Calculator

  1. 1

    Choose metric (cm, kg) or imperial (ft/in, lb) units.

  2. 2

    Select your sex and enter your height and body weight.

  3. 3

    Optionally enter a known body-fat percentage for a direct calculation.

  4. 4

    Compare the Boer, James and Hume estimates in the results table.

  5. 5

    Read your lean body mass, fat mass and lean mass index.

About the Lean Body Mass Calculator

The ByteTools Lean Body Mass Calculator works out how much of your body weight is not fat: muscle, bone, organs, connective tissue and body water. It runs the three standard prediction formulas — Boer, James and Hume — side by side from your height, weight and sex, so you can see the spread rather than trusting one estimate.

If you already know your body-fat percentage from a scan, a caliper reading or the US Navy tape method, enter it and the tool computes lean mass directly, which is far more reliable than any height-and-weight prediction. It also reports fat mass and your lean mass index.

Metric and imperial units are both supported, and every figure is computed locally in your browser with nothing uploaded or stored. These are estimates only, not medical advice — prediction formulas were fitted to general populations and can be well off for very muscular, very lean or very heavy individuals.

Frequently asked questions

What is lean body mass?

Lean body mass is your total body weight minus all body fat. It includes muscle, bone, organs, skin, connective tissue and the water inside them. It is not the same as muscle mass — skeletal muscle is only about half of it.

What is the Boer formula for lean body mass?

For men, LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. For women, LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. An 80 kg man at 180 cm comes out at 61.4 kg of lean mass.

How accurate are lean body mass formulas?

Only moderately. Boer, James and Hume were each regression-fitted to different reference populations and they can disagree with each other by several kilograms for the same person — which is a fair signal of their true error. They assume an average body composition for your height and weight, so a bodybuilder and a sedentary person of identical size get the same answer. If you have a real body-fat measurement, use that instead.

Why do I need lean body mass?

It is the input to the Katch-McArdle metabolic equation, it is used for weight-based drug dosing in clinical settings, and it is the number to watch during a diet — losing weight while holding lean mass steady is the goal, and scale weight alone cannot tell you whether that is happening.

How do I calculate lean mass from body fat percentage?

Multiply your weight by (100 − body fat %) ÷ 100. At 80 kg and 20 percent body fat, lean mass is 80 × 0.80 = 64 kg and fat mass is 16 kg. This is exact arithmetic — its accuracy depends entirely on how good your body-fat measurement is.

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