{
  "slug": "relative-fat-mass",
  "title": "Relative Fat Mass (RFM) Calculator",
  "category": "Body Composition",
  "category_slug": "body-composition",
  "tier": 2,
  "summary": "Estimate body-fat percentage from height and waist circumference using the RFM equation.",
  "description": "This calculator estimates whole-body fat percentage using the Relative Fat Mass (RFM) equation published by Woolcott and Bergman in 2018: RFM = 64 − 20 × (height ÷ waist) + 12 × sex, where the sex term is 12 for women and 0 for men. The equation was derived and validated against DXA scans in the NHANES population and needs only two tape-friendly inputs — height and waist circumference — making it the simplest published body-fat estimator on this site. The output is a population-level estimate, not a measurement of any individual.",
  "page_url": "https://fitmetriclab.com/en/tools/body-composition/relative-fat-mass/",
  "api_url": "https://fitmetriclab.com/api/tools/relative-fat-mass.json",
  "formula": {
    "id": "relative_fat_mass",
    "expression": "\\text{RFM} = 64 - 20 \\times \\frac{h}{w} + 12 \\times s",
    "variables": [
      {
        "symbol": "h",
        "description": "Height in cm"
      },
      {
        "symbol": "w",
        "description": "Waist circumference in cm"
      },
      {
        "symbol": "s",
        "description": "1 for female, 0 for male"
      }
    ]
  },
  "inputs": [
    {
      "id": "height_cm",
      "label": "Height",
      "type": "number",
      "unit": "cm",
      "default": 175,
      "min": 120,
      "max": 230,
      "step": 0.5
    },
    {
      "id": "waist_cm",
      "label": "Waist Circumference",
      "type": "number",
      "unit": "cm",
      "default": 85,
      "min": 40,
      "max": 200,
      "step": 0.5
    },
    {
      "id": "sex",
      "label": "Sex",
      "type": "select",
      "unit": null,
      "default": 1,
      "options": [
        {
          "label": "Male",
          "value": 1
        },
        {
          "label": "Female",
          "value": 0
        }
      ]
    }
  ],
  "worked_example": {
    "inputs": {
      "height_cm": 175,
      "waist_cm": 85,
      "sex": 1
    },
    "expected_output": "22.8%"
  },
  "result_summary_template": "At {height_cm} cm tall with a waist of {waist_cm} cm, the RFM equation estimates body fat at {result}.",
  "methodology": "Relative Fat Mass = 64 − 20 × (height ÷ waist circumference) + 12 × sex, with height and waist in the same units and sex = 1 for female, 0 for male (Woolcott & Bergman, 2018). The equation was selected from 365 candidate indices by fit against DXA-measured whole-body fat percentage in NHANES 1999–2006 participants aged 20+, then validated in a separate NHANES sample. The result is clamped to the 0–100% range and reported to one decimal place.",
  "sources": [
    {
      "label": "Woolcott OO, Bergman RN. Relative fat mass (RFM) as a new estimator of whole-body fat percentage — A cross-sectional study in American adult individuals. Sci Rep. 2018;8(1):10980.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/30030479/"
    }
  ],
  "faqs": [
    {
      "question": "How was the RFM equation derived and validated?",
      "answer": "Woolcott and Bergman evaluated 365 candidate indices built from basic anthropometrics against whole-body DXA scans from NHANES 1999–2004 participants, looking for the simplest function that tracked measured fat percentage. Height divided by waist circumference performed best, and the linear equation fitted to it — 64 − 20 × (height ÷ waist) + 12 × sex — was then validated in an independent NHANES 2005–2006 sample of around 4,000 adults. In validation, RFM agreed with DXA more closely than BMI did in both sexes and produced fewer large individual misestimates."
    },
    {
      "question": "Why does the equation add 12 for women?",
      "answer": "Because at the same body geometry, women in the derivation data carried systematically more fat than men — a combination of higher essential fat and different fat distribution. The regression found that offset to average about 12 percentage points across the NHANES sample, so the equation encodes it as a constant rather than fitting separate slopes. It is a population average, not an individual truth: any specific woman or man can sit above or below the offset, which is one of the ways individual estimates drift from DXA."
    },
    {
      "question": "How does RFM compare with the US Navy method?",
      "answer": "Both are regression equations fitted against a laboratory reference — DXA for RFM, hydrostatic weighing for the Navy method — but they take different inputs. The Navy method uses height, waist, and neck (plus hips for women) combined through logarithms; RFM uses only height and waist in a single linear expression. The Navy method's extra girths give it some ability to adjust for build, while RFM trades that for fewer, more repeatable measurements. On the same person the two estimates commonly land within a few percentage points, and neither has authority over the other — they are different fits to different reference data."
    },
    {
      "question": "Where should the waist be measured for RFM?",
      "answer": "The equation was fitted to NHANES waist measurements, which place the tape at the top of the iliac crest — the upper edge of the hip bone — horizontal to the floor, snug without compression, read at the end of a normal exhalation. That landmark sits lower than the WHO's rib-to-crest midpoint convention and can differ from it by a few centimetres on the same person. Since 1 cm of waist moves the RFM estimate by roughly half a percentage point, using the NHANES landmark, and using it consistently, keeps the input aligned with what the equation was actually fitted to."
    },
    {
      "question": "Can RFM replace a DXA scan or other lab measurement?",
      "answer": "No. RFM predicts what a DXA scan would likely report for a person of given height, waist, and sex in a population resembling NHANES — it is a statistical stand-in, not a measurement. Individual deviations of several percentage points are normal, and larger ones occur at extremes of leanness, girth, age, or stature, or in populations unlike the derivation sample. What the equation offers is a defensible tape-only estimate with published error characteristics, useful for tracking direction over time with consistent technique rather than for pinning down an exact figure."
    }
  ],
  "related_tools": [
    {
      "slug": "body-fat-navy",
      "title": "Body Fat Percentage (Navy Method)",
      "api_url": "https://fitmetriclab.com/api/tools/body-fat-navy.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/body-fat-navy/"
    },
    {
      "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/"
    },
    {
      "slug": "waist-to-height-ratio",
      "title": "Waist-to-Height Ratio Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/waist-to-height-ratio.json",
      "page_url": "https://fitmetriclab.com/en/tools/body-composition/waist-to-height-ratio/"
    }
  ],
  "last_updated": "2026-08-08",
  "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."
}