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BUN/Creatinine Ratio Calculator

Get your BUN-to-creatinine ratio in one step.

How it is calculated

The ratio is a single division of two blood test values: Ratio = BUN / Cr. BUN is blood urea nitrogen and Cr is serum creatinine, both in mg/dL, which is the unit used in US lab reports. The result is a plain number without a unit, shown with one decimal, and it is read as BUN : 1, so 23.3 means 23.3 : 1. The tool only accepts values in a plausible lab range: BUN above 0 and up to 500 mg/dL, creatinine from 0.1 to 30 mg/dL. Anything outside that, including a BUN of 0, shows no result. The tool does not convert units and does not judge the result. If your lab reports urea in mmol/L or creatinine in µmol/L, those numbers are not interchangeable with mg/dL and must be converted first. Reference ranges differ by laboratory, and interpretation needs the clinical picture.

Worked example

A lab report shows BUN 28 mg/dL and creatinine 1.2 mg/dL. Divide BUN by creatinine: 28 / 1.2 = 23.3. Written as a ratio, that is about 23.3 : 1. A value above the range of 10 to 20 given in the general guidance may prompt a clinician to look at hydration or blood flow to the kidneys, but one ratio never makes a diagnosis. The creatinine and BUN values themselves, and the patient's history, matter just as much.

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Common mistakes

  • Entering urea in mmol/L or creatinine in µmol/L. A creatinine of 80 µmol/L is outside the accepted 0.1 to 30 range and shows no result; convert both values to mg/dL first.
  • Swapping the two fields. Typing creatinine as BUN divides a small number by a large one, and the ratio comes out far below 1.
  • Mixing up BUN and urea. BUN counts only the nitrogen part of urea, so a lab that reports plain urea in mg/dL gives a number about 2.14 times larger.
  • Treating the ratio as a kidney score. It compares two values and does not replace eGFR, and a normal ratio can occur with poor kidney function.
  • Typing 0 for BUN to see what happens, or a creatinine below 0.1. Both are rejected because they are not real lab values.

Sources

About this calculator

The BUN/creatinine ratio compares two waste products the kidneys filter out of the blood: blood urea nitrogen (BUN), a byproduct of protein breakdown, and creatinine, a byproduct of muscle metabolism. You get it by simply dividing BUN by creatinine (both in mg/dL), and it is one of the oldest bedside tools for figuring out why a patient's kidney values are abnormal, not just that they are.

A normal ratio sits around 10:1 to 20:1. A ratio above roughly 20:1 points toward prerenal azotemia — the kidneys themselves are fine, but reduced blood flow from dehydration, heart failure, or gastrointestinal bleeding causes the tubules to reabsorb disproportionately more urea than creatinine. A ratio below about 10:1 suggests intrinsic renal disease (acute tubular necrosis, for example) or can reflect low urea production from malnutrition or liver disease.

Emergency physicians, nephrologists, and internists use this ratio alongside eGFR, urinalysis, and the patient's history to quickly triage the likely cause of an elevated BUN or creatinine before ordering more invasive workups. Divide the BUN value by the creatinine value from the same lab report and the ratio appears straight away.

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