BUN/Creatinine Ratio Calculator

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. This calculator just takes your two lab values and returns the ratio instantly.