{
  "id": "bun-creatinine-ratio",
  "version": "619aeaefd2d2",
  "status": "published",
  "name": "BUN Creatinine Ratio Calculator",
  "question": "What is my BUN creatinine ratio?",
  "summary": "Divides blood urea nitrogen (BUN) by serum creatinine, or urea by creatinine in SI units, and shows whether the ratio is in the usual 10 to 20 range.",
  "category": "health",
  "subcategory": "clinical",
  "url": "https://www.acalculator.org/health/bun-creatinine-ratio-calculator",
  "markdown": "https://www.acalculator.org/health/bun-creatinine-ratio-calculator.md",
  "kind": "function",
  "method": "Ratio = BUN (mg/dL) ÷ creatinine (mg/dL). SI: BUN = urea (mmol/L) × 2.8014; creatinine (mg/dL) = µmol/L ÷ 88.4; SI ratio = urea (mmol/L) ÷ creatinine (mmol/L).",
  "assumptions": [
    "BUN and creatinine from the same blood sample.",
    "The ratio is rounded to 1 decimal and read from the rounded value: under 10 low, 10 to 20 usual, over 20 high.",
    "The ratio alone does not show the cause; it is read with the separate BUN and creatinine values."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "units": {
        "title": "Units",
        "description": "US units (BUN and creatinine in mg/dL) or SI units (urea in mmol/L, creatinine in µmol/L).",
        "type": "string",
        "enum": [
          "us",
          "si"
        ]
      },
      "bun": {
        "title": "BUN (mg/dL)",
        "description": "Blood urea nitrogen, in mg/dL.",
        "type": "number",
        "minimum": 1,
        "maximum": 300
      },
      "cr": {
        "title": "Creatinine (mg/dL)",
        "description": "Serum creatinine, in mg/dL.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 20
      },
      "urea": {
        "title": "Urea (mmol/L)",
        "description": "Serum urea, in mmol/L.",
        "type": "number",
        "minimum": 0.36,
        "maximum": 107
      },
      "crSi": {
        "title": "Creatinine (µmol/L)",
        "description": "Serum creatinine, in µmol/L.",
        "type": "number",
        "minimum": 8.84,
        "maximum": 1768
      }
    }
  },
  "outputs": {
    "ratio": {
      "label": "BUN:creatinine ratio",
      "description": "BUN (mg/dL) divided by creatinine (mg/dL), to 1 decimal.",
      "format": "number"
    },
    "range": {
      "label": "Range",
      "description": "Whether the ratio is low, in the usual range, or high.",
      "format": "text"
    },
    "siRatio": {
      "label": "Urea:creatinine ratio (SI)",
      "description": "Urea (mmol/L) divided by creatinine (mmol/L), the SI form of the ratio, to a whole number.",
      "format": "integer"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "units": "us",
      "bun": 18,
      "cr": 1,
      "urea": 6.4,
      "crSi": 88
    },
    "outputs": {
      "ratio": 18,
      "range": "Usual range (10 to 20)",
      "siRatio": 73
    },
    "text": "The BUN to creatinine ratio is 18: Usual range (10 to 20)."
  },
  "examples": [
    {
      "given": {
        "units": "us",
        "bun": 18,
        "cr": 1
      },
      "expect": {
        "ratio": 18,
        "range": "Usual range (10 to 20)",
        "siRatio": 73
      },
      "source": "18 ÷ 1.0 = 18; SI: 18 ÷ 2.8014 = 6.4254 mmol/L urea, 1.0 × 88.4 = 88.4 µmol/L, 6.4254 ÷ 0.0884 = 72.7, shown as 73 (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "us",
        "bun": 40,
        "cr": 1.2
      },
      "expect": {
        "ratio": 33.3,
        "range": "High (over 20)"
      },
      "source": "40 ÷ 1.2 = 33.33, over 20 (Hosten 1990) (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "si",
        "urea": 5,
        "crSi": 176.8
      },
      "expect": {
        "ratio": 7,
        "range": "Low (under 10)",
        "siRatio": 28
      },
      "source": "5 × 2.8014 = 14.007 mg/dL BUN, 176.8 ÷ 88.4 = 2.0 mg/dL, 14.007 ÷ 2.0 = 7.0; SI 5 ÷ 0.1768 = 28.3 (hand calculation in content.mdx)"
    },
    {
      "given": {
        "units": "us",
        "bun": 20,
        "cr": 1
      },
      "expect": {
        "ratio": 20,
        "range": "Usual range (10 to 20)"
      },
      "source": "20 ÷ 1.0 = 20.0, the top of the usual range (hand calculation)"
    }
  ],
  "sources": [
    "Hosten AO. BUN and creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; 1990. Chapter 193. https://www.ncbi.nlm.nih.gov/books/NBK305/",
    "KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150. https://kdigo.org/guidelines/ckd-evaluation-and-management/ (1 mg/dL creatinine = 88.4 µmol/L)",
    "IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard atomic weights: nitrogen 14.007. https://ciaaw.org/nitrogen.htm"
  ],
  "related": [],
  "changelog": [
    {
      "date": "2026-09-29",
      "note": "Published after this site's editorial review against the cited sources (no clinician review); the page notice says so."
    }
  ]
}