BUN/Creatinine Ratio Calculator

The standard reference range is roughly 10:1 to 20:1 when both values are in mg/dL, the US lab convention. Most healthy adults sit between 12 and 16. The ratio is a derived number, not a measured one: a value inside this band is consistent with normal kidney perfusion and normal urea handling, but it cannot rule kidney disease in or out by itself. Reference ranges vary slightly between labs, so always check against the band printed on your own report.

An elevated ratio is the classic signature of a pre-renal pattern. The most common drivers are volume depletion (vomiting, diarrhea, diuretics, poor oral intake), dehydration, congestive heart failure with low cardiac output, and renal hypoperfusion from sepsis or shock before the tubules themselves are injured. Upper GI bleeding pushes the ratio much higher (often well above 30) because absorbed blood proteins act as a nitrogen load. A high-protein diet, corticosteroids, and catabolic states such as severe burns or trauma can also drive the ratio up without true kidney injury.

A low ratio points away from pre-renal causes and toward conditions that either lower BUN or raise creatinine relatively. Reduced urea synthesis in advanced liver disease is the textbook cause, since the liver makes urea from ammonia. Low protein intake, SIADH (dilutional drop in BUN), pregnancy (increased GFR and plasma volume), and rhabdomyolysis (sharp creatinine rise from muscle breakdown) all push the ratio below 10. Acute tubular necrosis tends to keep the ratio in the 10 to 15 range, which is one reason the ratio helps separate intrinsic from pre-renal AKI.

BUN and creatinine each move with several factors, and looking at them in isolation hides patterns. Creatinine reflects glomerular filtration but is also driven by muscle mass and creatine intake; BUN reflects urea production and tubular reabsorption, which are sensitive to hydration, protein intake, GI bleeding, and liver function. The ratio amplifies the imbalance between the two. A patient with stable creatinine and a sudden jump in BUN looks normal on either lab alone but shows a clear ratio shift, which is often the first hint of dehydration or an upper GI bleed.

No. The ratio is a screening signal that points toward a category of cause (pre-renal vs intrinsic-renal, hydration vs liver dysfunction vs GI bleed). It is never a diagnosis on its own. Two patients with identical ratios can have completely different conditions, and the ratio can also be normal during real kidney injury. Any abnormal value must be interpreted alongside the full clinical picture: vital signs, hydration status, urine output, medications, dietary protein, liver function, and the trajectory of the labs across days. Use this calculator to do the arithmetic, not to make the call.