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Lean body mass calculator — Boer, James & Hume formulas.

Estimate your lean body mass from height and weight using three peer-reviewed clinical formulas. See results side by side, your estimated body-fat percentage, and how LBM is used in drug dosing, nutrition, and fitness tracking.

How it worksReal-time

Inputs

Your measurements

cm
kg
Height
175 cm
In imperial
5' 9"
Body weight
75 kg

Lean body mass

3-formula average

57.2kg

Mean of Boer, James, and Hume formulas · male

Body composition estimate76.3% lean · 23.7% fat
Lean body mass: 57.2 kgFat mass: 17.8 kg

Formula comparison

LBM by formula

male · kg
  • Boer1984
    58.1kg

    Validated against DEXA scanning, the most accurate non-invasive body-composition reference. Most widely cited for clinical LBM estimation.

  • James1976
    59kg

    Originally developed to estimate total body water for pharmacokinetic drug dosing. Still used in anaesthesia and ICU settings for dosing adjustments.

  • Hume1966
    54.5kg

    One of the earliest LBM regression formulas, derived from healthy British adults using isotope dilution. A foundational reference in clinical pharmacology.

Body weight
75 kg
165 lb
Lean body mass
57.2 kg
126.1 lb
Fat mass (est.)
17.8 kg
39.2 lb
Body fat % (est.)
23.7%
from average LBM formula

Clinical context

How LBM is used in practice

Drug dosing

Many medications, particularly antibiotics, anaesthetics, and chemotherapy agents; they are dosed to LBM rather than total body weight to avoid over-dosing in obese patients or under-dosing in muscular ones.

Fitness tracking

Tracking LBM over time reveals whether training and nutrition changes are building lean tissue or just shifting body-weight. Total weight alone is misleading when fat is being lost while muscle is gained.

Nutrition targets

Protein needs for muscle retention and hypertrophy are often expressed per kilogram of LBM (typically 1.6–2.2 g/kg LBM/day) rather than per kilogram of body weight, to account for varying body-fat levels.

Metabolic rate

Basal metabolic rate correlates strongly with LBM because lean tissue burns significantly more energy than fat. LBM is a key input in the Katch-McArdle BMR formula, which tends to be the most accurate BMR estimate.

Body fat % shown above is an estimate derived from the average LBM formula, not from a direct measurement method. For clinical-grade accuracy, use DEXA scanning, hydrostatic weighing, or the US Navy body-fat calculator.

Field guide

What lean body mass is and why it matters more than total weight.

What is lean body mass?

Lean body mass (LBM) is everything your body is made of except stored fat. It includes skeletal muscle, bone mineral, organs, connective tissue, blood, and total body water. It is sometimes called fat-free mass (FFM), though the terms are used interchangeably in most clinical and fitness contexts.

Understanding your LBM matters because total body weight is a blunt tool. Two people can weigh exactly 80 kg — one with 20% body fat, one with 32%. Their metabolic rate, protein needs, and drug dosing requirements are entirely different. LBM is the number that explains those differences.

LBM vs muscle mass

LBM is not the same as skeletal muscle mass. Muscle is the largest single component of LBM in most adults (roughly 40–50% of LBM), but LBM also includes:

  • Bone: about 15% of LBM in a healthy adult
  • Organs: liver, heart, kidneys, etc. (~8%)
  • Total body water: 60–70% of LBM in men, slightly less in women
  • Connective tissue, skin, and residual

This distinction is important: building muscle increases LBM, but so does gaining bone density or even significant edema (fluid retention). Conversely, losing muscle while losing fat can hold LBM roughly constant even as body composition improves.

The three formulas: Boer, James, and Hume

All three formulas are linear regressions of the form LBM = a × weight + b × height + c, fitted against cohorts of healthy adults measured by a reference technique (usually isotope dilution or DEXA):

Boer (1984)

Men: LBM = 0.407 × W(kg) + 0.267 × H(cm) − 19.2 Women: LBM = 0.252 × W(kg) + 0.473 × H(cm) − 48.3

The Boer equation was validated against DEXA scanning, which is considered the gold standard for body composition outside of a research lab. It is the most commonly cited formula in modern clinical and sports-science literature and tends to give the most conservative (slightly lower) LBM estimates compared to James and Hume.

James (1976)

Men: LBM = 1.1 × W − 128 × (W / H)² Women: LBM = 1.07 × W − 148 × (W / H)²

The James formula was developed to estimate the apparent volume of distribution of drugs that distribute into lean tissue rather than fat. Its quadratic (W/H)² term means it becomes less accurate at high body weights, the formula can produce values below the Boer formula for individuals with a BMI above ~30. This calculator clamps the result to prevent negative or implausible outputs.

Hume (1966)

Men: LBM = 0.3281 × W + 0.3393 × H − 29.5336 Women: LBM = 0.29569 × W + 0.41813 × H − 43.2933

The Hume equation is one of the oldest validated LBM formulas, derived from isotope-dilution studies on healthy British adults in the 1960s. It is structurally similar to Boer but predates the availability of DEXA scanning. Hume typically produces results close to, but slightly different from — Boer for most healthy adults.

Sex differences in LBM

Women have approximately 10–15% more body fat at any given BMI compared to men, primarily due to sex hormones, reproductive tissue, and differences in fat distribution. As a result, LBM per kilogram of body weight is systematically lower in women. This is why all three formulas use different constants for each sex. It is not a health deficit — the additional fat in women includes essential hormonal and reproductive reserves.

LBM also declines with age in both sexes, a process called sarcopenia. After age 30, adults lose roughly 3–8% of muscle mass per decade, accelerating after 60. Resistance training is the most effective known intervention for slowing or reversing sarcopenia.

How to use LBM in practice

  • Protein targeting. The current evidence-based range for muscle retention and growth is 1.6–2.2 g of protein per kilogram of LBM per day — not per kilogram of total body weight. For a person with 60 kg LBM, that means 96–132 g of protein daily.
  • Katch-McArdle BMR. The most accurate basal metabolic rate formula uses LBM directly: BMR = 370 + (21.6 × LBM in kg). If you know your LBM, this outperforms the Mifflin-St Jeor formula at the extremes (very lean or very heavy individuals). Try our BMR calculator for the full comparison.
  • Progress tracking. During a body recomposition phase — losing fat while maintaining or gaining muscle. Your total weight may stay constant while your LBM rises. Tracking LBM over months is a more honest progress metric than tracking the scale.

Limitations of these formulas

All three formulas are population-level regressions. They carry an error of roughly ±3–5 kg at the individual level, meaning your true LBM could be several kilograms higher or lower than any formula predicts. Athletes, older adults, and individuals at the extremes of BMI are least well-served by these equations.

For the most accurate individual LBM measurement, use DEXA scanning (widely available at sports clinics), Bod Pod air displacement plethysmography, or underwater weighing. These methods have typical errors under 2%, compared to 5–10% for formula estimates.

Disclaimer

For informational and educational use only. Not medical advice. Consult a healthcare professional before making decisions based on body composition estimates.