Medical & Clinical
BUN Creatinine Ratio Calculator
Two cheap blood values answer an old clinical question: are the kidneys struggling — or just thirsty?
The BUN/creatinine ratio divides blood urea nitrogen by serum creatinine. A normal ratio is about 10:1 to 20:1. Ratios above 20:1 often point to pre-renal causes such as dehydration; ratios below 10:1 suggest liver or nutrition factors. Interpretation belongs to your clinician — use these numbers to ask better questions.
Normal: 10:1 – 20:1.
Above 20:1 → think dehydration.
Below 10:1 → liver or low protein.
Calculate Your BUN/Creatinine Ratio
BUN/Creatinine Ratio
What Is BUN/Creatinine Ratio?
Blood urea nitrogen (BUN) and creatinine are both filtered by the kidneys — but they behave differently when blood flow to the kidneys drops. Urea gets reabsorbed; creatinine does not.
That asymmetry is the trick: in dehydration, BUN climbs faster than creatinine, pushing the ratio above 20:1 even before kidney damage exists.
How It Is Calculated
Ratio = BUN (mg/dL) ÷ creatinine (mg/dL)
Example: Example: BUN 18, creatinine 1.0 → ratio 18:1 (normal)
Ratio interpretation
| Ratio | Direction |
|---|---|
| < 10:1 | Liver disease, low protein, over-hydration |
| 10:1 – 20:1 | Normal |
| > 20:1 | Pre-renal — dehydration, bleeding, heart failure |
Limitations
- Many factors move the ratio — GI bleeding, steroids, and high protein diets all raise BUN.
- Interpret with the actual values, not the ratio alone.
- eGFR (CKD-EPI) is the better kidney-function number; the ratio answers a different question.
Sources & Review
References used for this calculator’s formulas and thresholds:
BUN/Creatinine Ratio FAQ
What does a high BUN/creatinine ratio mean?
Above 20:1 classically suggests reduced kidney blood flow — dehydration is the most common cause.
What is a normal BUN/creatinine ratio?
About 10:1 to 20:1 when both values are in conventional units.
Does a normal ratio rule out kidney disease?
No — chronic kidney disease often keeps a normal ratio. Check eGFR and urine studies for kidney function.
Important disclaimer: Results are educational estimates, not a substitute for professional medical advice. Discuss your results with a qualified professional before making decisions.