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BUN/Creatinine Ratio Calculator

Calculate the BUN-to-creatinine ratio from BUN and creatinine in mg/dL or from urea (mmol/L) and creatinine (µmol/L), with the SI urea:creatinine ratio and what high or low values suggest.

Units

Urea (SI) or BUN (US)

Creatinine

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How it works

BUN/Cr = BUN (mg/dL) ÷ creatinine (mg/dL) · from SI: BUN = urea (mmol/L) × 2.8014, creatinine mg/dL = µmol/L ÷ 88.42 · SI urea:creatinine = urea (mmol/L) ÷ creatinine (µmol/L) × 1,000 ≈ 4 × BUN/Cr

Creatinine is filtered and not reabsorbed, while urea is partly reabsorbed in the tubules — more so when the kidneys are underperfused and urine flow is slow. So in prerenal states (dehydration, heart failure, bleeding into the gut, high protein load or steroids) BUN rises proportionally more than creatinine and the ratio climbs above 20. In intrinsic kidney injury both rise together and the ratio stays around 10–20; a low ratio suggests low urea production (malnutrition, liver disease) or extra creatinine (rhabdomyolysis). The ratio is a supporting clue, not a diagnosis.

Worked example

Urea 5.0 mmol/L and creatinine 80 µmol/L: BUN = 5.0 × 2.8014 = 14.0 mg/dL and creatinine = 80 ÷ 88.42 = 0.90 mg/dL, so BUN/creatinine = 14.0 ÷ 0.90 = 15.5, in the normal 10–20 range. The SI urea:creatinine ratio is 5.0 ÷ 80 × 1,000 = 63.

Sources: StatPearls: BUN — Blood Urea Nitrogen · Uchino S et al. Blood urea nitrogen to creatinine ratio in acute kidney injury. Nephrol Dial Transplant 2012;27:2264–70

Frequently asked questions

What is a normal BUN to creatinine ratio?

Usually 10:1 to 20:1. Above 20 suggests a prerenal cause such as dehydration or an upper GI bleed; below 10 suggests low protein intake, liver disease or rhabdomyolysis.

How do I calculate the ratio with SI units?

Convert urea in mmol/L to BUN by multiplying by 2.8 and creatinine in µmol/L to mg/dL by dividing by 88.4, then divide. UK labs often use the urea:creatinine ratio (urea mmol/L ÷ creatinine µmol/L × 1,000) instead, which is about four times the BUN/creatinine ratio; values above about 100 suggest a prerenal cause.

Why does an upper GI bleed raise the BUN/creatinine ratio?

Blood in the gut is digested as protein and the liver turns it into urea, while creatinine is unaffected, so BUN rises out of proportion. A ratio above 30 makes an upper GI source more likely.

Can the ratio be normal in kidney failure?

Yes. In intrinsic kidney disease BUN and creatinine usually rise together, so the ratio stays around 10–20 even though both values are high. Look at the eGFR, not the ratio, to judge kidney function.

Related calculators

BUN to Urea Converter (mg/dL to mmol/L)

Convert blood urea nitrogen (BUN, mg/dL) to urea in mmol/L or mg/dL and back: urea mmol/L = BUN × 0.357 and urea mg/dL = BUN × 2.14, with the usual adult reference range.

Creatinine Converter (mg/dL to µmol/L)

Convert serum or urine creatinine between mg/dL, µmol/L and mmol/L (1 mg/dL = 88.42 µmol/L), with typical adult reference ranges and a quick conversion chart.

FENa Calculator (Fractional Excretion of Sodium)

Fractional excretion of sodium from urine and serum sodium and creatinine (µmol/L, mmol/L or mg/dL) — prerenal (< 1%), indeterminate or intrinsic AKI (> 2%) — with the optional FEUrea for patients on diuretics, the renal failure index, urine/plasma creatinine and BUN/creatinine ratios, and a worked solution.

FEUrea Calculator (Fractional Excretion of Urea)

Fractional excretion of urea from urine and serum urea and creatinine — the diuretic-resistant alternative to FENa for separating prerenal AKI (< 35%) from acute tubular necrosis (> 50%).

eGFR Calculator (CKD-EPI 2021)

Estimate glomerular filtration rate (eGFR) from serum creatinine, age and sex using the race-free 2021 CKD-EPI equation, with the matching CKD stage.

Educational calculation only, not medical advice or a diagnosis. Reference ranges and units differ between laboratories; interpret any result with your doctor in the context of your full clinical picture.