About the BUN to Creatinine Ratio Calculator
The BUN to creatinine ratio compares blood urea nitrogen (BUN) with serum creatinine from the same blood sample. Both rise when kidney function falls, but urea is also reabsorbed by the kidney tubules and is produced in larger amounts after a high protein load or bleeding into the gut, so the ratio between them helps clinicians think about why the numbers are abnormal. A ratio above about 20:1 classically points towards a prerenal cause such as dehydration, heart failure or upper gastrointestinal bleeding, while a ratio inside 10–20:1 is typical of normal function or intrinsic kidney disease.
Enter BUN and creatinine in mg/dL (US units), or switch to SI units and enter urea in mmol/L and creatinine in µmol/L as reported in the UK, Europe and Australia. The calculator converts between systems, so you get both the US-style BUN/Cr ratio and the SI urea:creatinine ratio (urea and creatinine both in mmol/L), where roughly 40–80 corresponds to the conventional 10–20:1 and values above about 100 are traditionally read as prerenal.
The ratio is a supporting clue for students, nurses and clinicians reviewing a basic metabolic panel — not a diagnosis. In a 2017 study of 1,103 emergency patients with acute kidney injury, the ratio did not distinguish prerenal from intrinsic causes at all (area under the ROC curve 0.5). It is also unreliable when creatinine is very low (for example in frail older adults with little muscle mass) and should always be read with the absolute values, urine studies, the trend over time and the clinical picture.
With the default inputs, the bun/creatinine ratio is 20 : 1. Change any value above to recalculate instantly.
How to use the bun to creatinine ratio calculator
- 1Choose US units (mg/dL) or SI units (mmol/L and µmol/L) to match your lab report.
- 2Enter BUN or urea from the metabolic panel.
- 3Enter serum creatinine from the same sample.
- 4Read the ratio and its interpretation band.
- 5Interpret it alongside the absolute values, urine studies and the clinical picture.
Formula and method
The ratio is simply BUN divided by creatinine when both are in mg/dL. In SI units, urea in mmol/L is converted to urea nitrogen by multiplying by 2.8 (each urea molecule carries two nitrogen atoms, 28 mg per mmol), and creatinine in µmol/L is converted to mg/dL by dividing by 88.4.
The SI urea:creatinine ratio divides urea (mmol/L) by creatinine converted to mmol/L. Because of the unit conversions it is about 4 times the US BUN/Cr ratio, so the conventional 10–20:1 range corresponds to roughly 40–80, and many UK references treat values above about 100 as suggestive of a prerenal or upper GI bleeding cause.
- BUN
- Blood urea nitrogen (mg/dL)
- Cr
- Serum creatinine (mg/dL or µmol/L)
- urea
- Serum urea (mmol/L), the SI equivalent of BUN
Worked examples
Normal panel in US units
BUN 20 mg/dL ÷ creatinine 1.0 mg/dL = 20:1, the top of the conventional 10–20:1 range. In SI terms this is urea 7.1 mmol/L ÷ creatinine 0.0884 mmol/L ≈ 81.
Raised ratio suggesting a prerenal cause
BUN 48 ÷ creatinine 1.6 = 30:1. Both values are raised, but BUN is out of proportion to creatinine, the classic pattern of reduced kidney perfusion (for example dehydration) or an upper GI bleed.
SI units with a low ratio
Urea 5.0 mmol/L × 2.8 = BUN 14 mg/dL, and creatinine 150 µmol/L ÷ 88.4 = 1.70 mg/dL. The BUN/Cr ratio is 14 ÷ 1.70 ≈ 8.3:1 (SI urea:creatinine 5.0 ÷ 0.150 ≈ 33), below the usual range.
Frequently asked questions
What is a normal BUN to creatinine ratio?+
Most references give a normal BUN/creatinine ratio of about 10:1 to 20:1 when both values are measured in mg/dL. In SI units (urea and creatinine both in mmol/L) the equivalent range is roughly 40–80.
What does a high BUN/creatinine ratio mean?+
A ratio above 20:1 suggests that urea is rising faster than creatinine. Classic causes are prerenal states such as dehydration, heart failure or blood loss, upper gastrointestinal bleeding, a high protein diet, catabolic illness and corticosteroid treatment.
What does a low BUN/creatinine ratio mean?+
A ratio below 10:1 can occur with low protein intake or malnutrition, severe liver disease (reduced urea production), rhabdomyolysis (extra creatinine release), SIADH, pregnancy and after dialysis, which removes urea efficiently.
How do I convert urea in mmol/L to BUN in mg/dL?+
Multiply urea in mmol/L by 2.8 to get BUN in mg/dL (for example, 7 mmol/L × 2.8 ≈ 19.6 mg/dL). To convert creatinine from µmol/L to mg/dL, divide by 88.4.
How reliable is the BUN/creatinine ratio for diagnosing prerenal AKI?+
Less reliable than textbooks suggest. Manoeuvrier and colleagues (BMC Nephrology 2017) studied 1,103 emergency department patients with AKI: the mean SI ratio was about 91 in both prerenal and intrinsic groups, and at the usual threshold of 100 sensitivity was 70% and specificity only 33%.
Can the ratio be normal if kidney function is poor?+
Yes. In intrinsic kidney disease and urinary obstruction BUN and creatinine usually rise together, so the ratio often stays in the 10–20:1 range even though both values are high. Look at eGFR and the absolute values too.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.