BUN-to-Creatinine Ratio Calculator
Get the BUN/creatinine ratio in mg/dL or SI units and read what the result suggests about the kidneys.
Result
How it is calculated
The ratio is BUN ÷ creatinine, with both values in mg/dL. If you choose SI units, the engine first converts urea (mmol/L) with BUN = urea × 2.8 and creatinine (µmol/L) with creatinine mg/dL = µmol/L ÷ 88.4, then divides. The ratio is shown to one decimal place, and the converted values are echoed back so you can check them. The tool labels a ratio below 10 as low, 10 to 20 (20 included) as the usual range, and above 20 as a prerenal pattern. A creatinine of zero or below, or a negative BUN, returns no result; the tool does not set an upper limit, so a typo such as 0.01 mg/dL simply produces a huge ratio. These cut-offs are common teaching values; laboratories and clinical contexts differ.
Worked example
Take BUN 30 mg/dL and creatinine 1.2 mg/dL, with conventional units. No conversion is needed, so the ratio is 30 ÷ 1.2 = 25.0. The tool echoes the values used as BUN 30.0 mg/dL and creatinine 1.20 mg/dL. Because 25.0 is above 20, it says this suggests a prerenal cause such as dehydration, hypovolaemia, GI bleeding or heart failure. With SI input of urea 6 mmol/L and creatinine 110 µmol/L the same steps give 16.8 and 1.24 mg/dL, a ratio of 13.5, within the usual range.
Common mistakes
- Entering urea in mmol/L while leaving the units on conventional: 6 would be read as BUN 6 mg/dL instead of 16.8 mg/dL, giving a far too low ratio (5.0 instead of 13.5 in the example above).
- Typing creatinine in µmol/L (for example 110) in conventional mode; the ratio then collapses to 0.3, and no warning is given.
- Confusing BUN with urea: BUN is urea nitrogen, and the tool multiplies urea by 2.8 only in SI mode.
- Reading the ratio alone: a normal ratio with both values high is not reassuring, because the tool only compares the two numbers, not their absolute levels.
- Using unrealistic creatinine such as 0.01 mg/dL; the engine still divides and prints a ratio of 3,000.0 instead of warning.
About this calculator
The BUN-to-creatinine ratio divides blood urea nitrogen by serum creatinine to help distinguish between causes of kidney dysfunction: in conventional US units (mg/dL), a normal ratio runs roughly 10:1 to 20:1; SI results (urea in mmol/L, creatinine in µmol/L) are converted to mg/dL first, after which the same 10:1-to-20:1 reference range applies — the ratio is dimensionless, so only the input units differ between systems. Both markers rise when kidney function drops, but they don’t always rise at the same rate, and that gap is diagnostically useful.
A ratio elevated above the normal range (often greater than 20:1) points toward a prerenal cause — dehydration, heart failure, or reduced blood flow to the kidneys — where the kidney tissue itself is intact but under-perfused, causing urea to be reabsorbed disproportionately more than creatinine. A ratio within the normal range but with both values elevated suggests intrinsic kidney damage instead, while a low ratio can point toward liver disease, malnutrition, or a low-protein diet. Physicians use it alongside a full metabolic panel, not in isolation, to help narrow the differential for acute kidney injury.
This calculator divides your BUN by your creatinine value in either conventional or SI units and flags whether the result suggests a prerenal pattern.
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