{
  "slug": "lean-body-mass",
  "title": "Lean Body Mass Calculator",
  "category": "Body Composition",
  "category_slug": "body-composition",
  "tier": 2,
  "summary": "Calculate lean body mass by subtracting fat mass from total weight.",
  "description": "This calculator estimates lean body mass by subtracting fat mass from total body weight using a two-compartment body composition model. It requires weight in kilograms and body fat percentage as inputs, then outputs lean body mass in kilograms along with the corresponding fat mass. The method reflects the foundational definition used in exercise physiology and represents the sum of muscle, bone, organs, and all non-fat tissue, though it does not distinguish between these individual components.",
  "page_url": "https://fitmetriclab.com/en/tools/body-composition/lean-body-mass/",
  "api_url": "https://fitmetriclab.com/api/tools/lean-body-mass.json",
  "formula": {
    "id": "lean_body_mass",
    "expression": "\\text{LBM} = w \\times (1 - b)",
    "variables": [
      {
        "symbol": "w",
        "description": "Weight in kg"
      },
      {
        "symbol": "b",
        "description": "Body-fat percentage (decimal)"
      }
    ]
  },
  "inputs": [
    {
      "id": "weight_kg",
      "label": "Weight",
      "type": "number",
      "unit": "kg",
      "default": 75,
      "min": 20,
      "max": 300,
      "step": 0.1
    },
    {
      "id": "body_fat_percent",
      "label": "Body Fat",
      "type": "number",
      "unit": "%",
      "default": 20,
      "min": 2,
      "max": 70,
      "step": 0.1
    }
  ],
  "worked_example": {
    "inputs": {
      "weight_kg": 75,
      "body_fat_percent": 20
    },
    "expected_output": "60.0 kg"
  },
  "result_summary_template": "For a body weight of {weight_kg} kg and a body-fat percentage of {body_fat_percent}%, lean body mass is estimated at {result}.",
  "methodology": "LBM = Weight - (Weight × Body Fat %). The formula is a two-compartment model that partitions body mass into fat and fat-free components. It does not originate from a single study but reflects the foundational definition of lean body mass in exercise physiology and clinical body-composition literature.",
  "sources": [
    {
      "label": "Heymsfield SB, et al. Human body composition: advances in models and methods. Annu Rev Nutr. 1997;17:527-58.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/9240939/"
    },
    {
      "label": "Wang Z, et al. The five-level model: a new approach to organizing body-composition research. Am J Clin Nutr. 1992;56(1):19-28.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/1609756/"
    },
    {
      "label": "Lee SY, Gallagher D. Assessment methods in human body composition. Curr Opin Clin Nutr Metab Care. 2008;11(5):566-72.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/18685451/"
    }
  ],
  "faqs": [
    {
      "question": "What counts as lean body mass?",
      "answer": "Lean body mass includes skeletal muscle, bone mineral, organs, connective tissue, and body water—everything except adipose (fat) tissue. The calculator does not separate these components; it treats them as a single lean compartment."
    },
    {
      "question": "How accurate is this calculation?",
      "answer": "The arithmetic is exact. Accuracy depends entirely on the body-fat percentage input. DEXA and hydrostatic weighing offer ±1–2% precision, while skinfold calipers and bioelectrical impedance may vary by ±3–5% or more depending on technique and hydration."
    },
    {
      "question": "Can I use this to track muscle gain?",
      "answer": "Changes in lean body mass may reflect muscle gain, but they can also result from shifts in glycogen, water, bone density, or measurement error. Isolating muscle mass requires more detailed imaging (DEXA, MRI) or longitudinal trends alongside strength and performance data."
    },
    {
      "question": "Why does my lean body mass fluctuate day to day?",
      "answer": "Hydration, meal timing, sodium intake, and menstrual cycle phase all affect body water, which is part of lean mass. Bioelectrical impedance devices are especially sensitive to these factors. Consistent measurement conditions reduce session-to-session noise."
    },
    {
      "question": "Does this calculator work for children or older adults?",
      "answer": "The formula applies to any age group arithmetically, but body-fat measurement methods have different validity ranges. Skinfold equations are often population-specific (age, sex, ethnicity), and pediatric or geriatric norms may require specialized protocols or reference standards."
    }
  ],
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    {
      "slug": "bmr-harris-benedict",
      "title": "BMR Calculator (Harris-Benedict)",
      "api_url": "https://fitmetriclab.com/api/tools/bmr-harris-benedict.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/bmr-harris-benedict/"
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    {
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      "api_url": "https://fitmetriclab.com/api/tools/bmr-katch-mcardle.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/bmr-katch-mcardle/"
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    {
      "slug": "body-fat-3-site",
      "title": "Body Fat Percentage (3-Site Caliper)",
      "api_url": "https://fitmetriclab.com/api/tools/body-fat-3-site.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/body-fat-3-site/"
    }
  ],
  "last_updated": "2026-04-22",
  "price": {
    "value": 0,
    "currency": "USD"
  },
  "runs_in_browser": true,
  "disclaimer": "Educational estimate produced from a published formula. Not medical, nutritional, or training advice. Calculations are deterministic — apply the formula to the inputs to reproduce the result."
}