{
  "slug": "bmr-katch-mcardle",
  "title": "BMR Calculator (Katch-McArdle)",
  "category": "Body Composition",
  "category_slug": "body-composition",
  "tier": 2,
  "summary": "Estimate basal metabolic rate using lean body mass instead of age and sex.",
  "description": "This calculator estimates basal metabolic rate (BMR) using the Katch-McArdle equation, which relies on lean body mass rather than age or biological sex. It requires total body weight in kilograms and body fat percentage as inputs, then applies the formula BMR = 370 + (21.6 × lean body mass) to produce a daily resting energy expenditure in kilocalories. The method is most accurate when body fat percentage is measured reliably, as the equation derives lean mass by subtracting fat mass from total weight.",
  "page_url": "https://fitmetriclab.com/en/tools/body-composition/bmr-katch-mcardle/",
  "api_url": "https://fitmetriclab.com/api/tools/bmr-katch-mcardle.json",
  "formula": {
    "id": "bmr_katch_mcardle",
    "expression": "\\text{BMR} = 370 + 21.6 \\times \\text{LBM}",
    "variables": [
      {
        "symbol": "w",
        "description": "Weight in kg"
      },
      {
        "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": "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": "1,666 kcal"
  },
  "result_summary_template": "For a body weight of {weight_kg} and a body-fat percentage of {body_fat_percent}, the Katch-McArdle equation estimates basal metabolic rate at {result}.",
  "methodology": "Applies the Katch-McArdle equation: BMR = 370 + (21.6 × lean body mass in kg). Lean body mass is derived by multiplying total weight by (1 − body-fat % / 100). Originally published by Katch and McArdle in exercise-physiology texts using metabolic-chamber measurements.",
  "sources": [
    {
      "label": "Katch FI, McArdle WD. Introduction to Nutrition, Exercise, and Health. 4th ed. Lippincott Williams & Wilkins; 1993.",
      "url": "https://www.worldcat.org/title/introduction-to-nutrition-exercise-and-health/oclc/27430598"
    },
    {
      "label": "Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr. 1991;54(6):963-969.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/1957828/"
    },
    {
      "label": "Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr. 2005;82(5):941-948.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/16280423/"
    }
  ],
  "faqs": [
    {
      "question": "Why does Katch-McArdle ignore age and sex?",
      "answer": "The formula assumes that lean body mass alone determines resting metabolic rate. Age and sex correlate with BMR largely because they track changes in muscle mass and organ size; by measuring body composition directly, Katch-McArdle bypasses demographic proxies. Research shows lean mass explains most of the variance in BMR across individuals."
    },
    {
      "question": "How accurate does my body-fat measurement need to be?",
      "answer": "Each 1% error in body-fat percentage shifts lean mass by roughly 0.75 kg for a 75 kg person, altering BMR by about 16 kcal/day. A 5% measurement error moves the estimate by 80 kcal. DEXA, hydrostatic weighing, and multi-site skinfold protocols offer the tightest precision; single-frequency BIA scales can drift by 3–5% session to session."
    },
    {
      "question": "Can I use this formula if I am obese or very lean?",
      "answer": "The Katch-McArdle equation was derived from adults in moderate body-fat ranges. At very high body-fat percentages (above 40%), adipose tissue contributes more to resting metabolism than the constant 21.6 factor assumes, potentially underestimating BMR. Elite athletes below 8% body fat may have organ and metabolic adaptations not captured by the equation."
    },
    {
      "question": "What is the difference between BMR and TDEE?",
      "answer": "BMR is the energy required for basic physiological functions—heartbeat, respiration, cellular repair—measured at complete rest. Total daily energy expenditure (TDEE) adds physical activity, non-exercise movement, and the thermic effect of digesting food. TDEE is typically 1.2–2.5 times BMR, depending on lifestyle and training volume."
    },
    {
      "question": "How often does BMR change?",
      "answer": "BMR shifts whenever lean body mass changes. Gaining muscle through resistance training raises BMR proportionally; losing muscle during a calorie deficit or aging lowers it. Short-term factors like sleep deprivation, illness, or extreme dieting can also alter metabolic rate by 5–15%, though these effects are not captured by static body-composition equations."
    }
  ],
  "related_tools": [
    {
      "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/"
    },
    {
      "slug": "ffmi-calculator",
      "title": "FFMI Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/ffmi-calculator.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/ffmi-calculator/"
    },
    {
      "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/"
    }
  ],
  "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."
}