{
  "slug": "ffmi-calculator",
  "title": "FFMI Calculator",
  "category": "Body Composition",
  "category_slug": "body-composition",
  "tier": 2,
  "summary": "Calculate Fat-Free Mass Index normalized for height using weight, height, and body-fat percentage.",
  "description": "This calculator computes Fat-Free Mass Index (FFMI) using the normalized equation from Kouri et al. (1995), which adjusts lean body mass relative to height squared with a standardization constant. It requires weight, height, and body-fat percentage as inputs and returns both a height-adjusted FFMI score and the raw unadjusted value. The normalized FFMI allows for comparison of muscle mass across individuals of different heights and is often used to assess natural muscular development potential.",
  "page_url": "https://fitmetriclab.com/en/tools/body-composition/ffmi-calculator/",
  "api_url": "https://fitmetriclab.com/api/tools/ffmi-calculator.json",
  "formula": {
    "id": "ffmi",
    "expression": "\\text{FFMI} = \\frac{\\text{LBM}}{h^{2}} + 6.1 \\times (1.8 - h)",
    "variables": [
      {
        "symbol": "w",
        "description": "Weight in kg"
      },
      {
        "symbol": "h",
        "description": "Height in metres"
      },
      {
        "symbol": "b",
        "description": "Body-fat percentage"
      }
    ]
  },
  "inputs": [
    {
      "id": "weight_kg",
      "label": "Weight",
      "type": "number",
      "unit": "kg",
      "default": 75,
      "min": 20,
      "max": 300,
      "step": 0.1
    },
    {
      "id": "height_cm",
      "label": "Height",
      "type": "number",
      "unit": "cm",
      "default": 175,
      "min": 100,
      "max": 230,
      "step": 0.5
    },
    {
      "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,
      "height_cm": 175,
      "body_fat_percent": 20
    },
    "expected_output": "19.9"
  },
  "result_summary_template": "With a weight of {weight_kg} kg, height of {height_cm} cm, and body fat of {body_fat_percent}%, the normalized FFMI is {result}.",
  "methodology": "Uses normalized FFMI equation: FFMI = LBM(kg) / height(m)² + 6.1 × (1.8 - height in m), where LBM = weight × (1 - body_fat% / 100). Normalization constant 6.1 and reference height 1.8 m derived from Kouri et al. (1995) to standardize comparisons across heights.",
  "sources": [
    {
      "label": "Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223-8.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/7496846/"
    },
    {
      "label": "Schutz Y, Kyle UU, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. Int J Obes Relat Metab Disord. 2002;26(7):953-60.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/12080449/"
    },
    {
      "label": "VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA. Height-normalized indices of the body's fat-free mass and fat mass: potentially useful indicators of nutritional status. Am J Clin Nutr. 1990;52(6):953-9.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/2239792/"
    }
  ],
  "faqs": [
    {
      "question": "What does the FFMI normalization adjustment do?",
      "answer": "The term \"6.1 × (1.8 - height in m)\" adjusts the raw FFMI so that individuals of different heights can be compared on the same scale. Without this correction, taller individuals would systematically score lower and shorter individuals higher, even with identical muscle mass relative to their frame."
    },
    {
      "question": "Why does body-fat percentage matter for FFMI?",
      "answer": "FFMI measures lean (fat-free) mass relative to height, not total body weight. Body-fat percentage is required to partition total weight into fat mass and lean mass. A 90 kg individual at 10% body fat has far more lean mass than a 90 kg individual at 30% body fat."
    },
    {
      "question": "What FFMI values are typical?",
      "answer": "Published cohort studies show untrained adult males typically range from 17 to 20, trained natural athletes from 20 to 23, and elite bodybuilders above 25. These are population observations, not thresholds or diagnostic cutoffs. Individual variation exists due to genetics, training history, and measurement accuracy."
    },
    {
      "question": "Can I use FFMI to track progress over time?",
      "answer": "FFMI can serve as a progress metric if body-fat percentage is measured consistently using the same method at similar times of day. Changes in hydration, glycogen, and measurement technique can introduce session-to-session noise. Trends over months are more informative than single-session comparisons."
    },
    {
      "question": "How does FFMI differ from BMI?",
      "answer": "BMI divides total body weight by height squared without distinguishing fat from lean mass. FFMI isolates lean mass before the height adjustment, making it more specific to muscularity. A high-BMI individual may have high FFMI (muscular) or low FFMI (higher body fat)."
    }
  ],
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      "page_url": "https://fitmetriclab.com/en/tools/body-composition/bmr-harris-benedict/"
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    {
      "slug": "lean-body-mass",
      "title": "Lean Body Mass Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/lean-body-mass.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/lean-body-mass/"
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  ],
  "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."
}