Divide BUN by creatinine and read the classical pattern bands (> 20, 10–20, < 10) with honest context for each lab value.
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Educational tool only — not medical advice.
The BUN/creatinine ratio is pattern information from the medical literature, not a diagnosis of kidney disease. It cannot confirm or rule out any condition. Always discuss kidney test results with a qualified clinician.
Enter your lab values
Both values are converted to mg/dL internally before dividing — the ratio is unitless.
Try a worked example (all recomputed by the engine):
Please enter positive numbers for BUN and creatinine (both must be > 0).
Your BUN/creatinine ratio
30.00
Prerenal pattern — ratio > 20
BUN
30.0 mg/dL · 10.71 mmol/L
Creatinine
1.00 mg/dL · 88.4 µmol/L
How it was computed
Pattern information, not a diagnosis.
This band describes what the medical literature classically associates with this ratio pattern. It does not diagnose prerenal azotemia, intrinsic kidney disease, or any other condition — and it cannot rule any of them out.
Cutoff variants — the honest footnote
The upper boundary of 20 is consistent across sources (StatPearls, renal teaching tables). The lower boundary varies:
Source
Prerenal
Normal / typical
Intrarenal
This tool
> 20
10–20
< 10
StatPearls (NCBI)
> 20:1
—
< 20:1
Wikipedia (urea:Cr)
> 20:1
12–20:1
< 12:1
Borderline values (around 10–12 or exactly 20) sit in a gray zone between references — another reason the ratio is always read in clinical context, never alone. Post-renal causes also fall in the middle range, so the ratio cannot separate those either.
When is the ratio worth reading — and when is it not?
Both values in the same units. This calculator converts for you — BUN × 0.357 = mmol/L, creatinine × 88.4 = µmol/L. Mixed units silently corrupt the ratio.
Read alongside the absolute values. A normal ratio with both values elevated still signals kidney involvement — intrinsic injury classically keeps the ratio around 10–15.
Check for confounders first: high protein diet, GI bleeding, corticosteroids, tetracyclines, dehydration (raise BUN); liver disease, malnutrition, muscle mass (shift either).
Never as a diagnosis. It is one hint among history, full labs, urine findings and validated kidney assessments.
The BUN/creatinine ratio is calculated by dividing blood urea nitrogen (BUN) by serum creatinine, both measured in the same units (mg/dL). It compares two waste products the kidneys filter, and clinicians use it as one clue — never alone — about whether elevated kidney markers reflect reduced blood flow to the kidneys or a problem within the kidneys themselves.
What is a normal BUN/creatinine ratio?
A BUN/creatinine ratio between 10 and 20 is generally considered typical for adults. A ratio above 20 is the classical pattern described with prerenal causes such as dehydration or reduced renal blood flow, while a ratio below 10 is the classical pattern described with intrinsic kidney disease. Note that some references use slightly different cutoffs (for example 12 instead of 10 for the low boundary), so borderline values are always read in clinical context.
What does a high BUN/creatinine ratio (above 20) suggest?
A ratio above 20 means BUN is disproportionately high relative to creatinine. The medical literature classically associates this pattern with prerenal azotemia — conditions where blood flow to the kidneys drops, such as dehydration or heart failure. Importantly, the same high ratio can be produced without any kidney problem at all: gastrointestinal bleeding (digested blood releases urea), high protein intake, and corticosteroid medications all raise BUN while creatinine stays roughly normal.
What does a low BUN/creatinine ratio (below 10) suggest?
A ratio below 10 means creatinine is disproportionately high relative to BUN, which the medical literature classically associates with intrinsic (intrarenal) kidney disease, where both markers rise more proportionally. However, a low ratio also occurs with liver disease (because urea is manufactured in the liver), malnutrition or low protein intake, and overhydration. The ratio alone cannot distinguish these possibilities.
How do I convert BUN from mg/dL to mmol/L, and creatinine from mg/dL to µmol/L?
To convert BUN from mg/dL to mmol/L (as urea), multiply by 0.357 — so 30 mg/dL becomes 10.71 mmol/L. To convert creatinine from mg/dL to µmol/L, multiply by 88.4 — so 1.0 mg/dL becomes 88.4 µmol/L. This calculator accepts both unit systems and converts internally; the ratio itself is unitless as long as both inputs use the same units.
Can the BUN/creatinine ratio diagnose kidney failure?
No. The ratio is traditional pattern information, not a diagnosis. Modern studies of acute kidney injury have found that a ratio above 20 does not reliably separate prerenal from intrinsic kidney injury on its own, and many patients with kidney injury have ratios in the normal range. A clinician interprets the ratio alongside the absolute values, urine findings, medications, diet, hydration status, and the full clinical picture.
What can make the ratio misleading?
Several common factors move the ratio without reflecting kidney location. BUN can be raised by high protein intake, gastrointestinal bleeding, corticosteroids, tetracyclines, and dehydration. BUN can be lowered by liver disease and malnutrition. Creatinine depends on muscle mass, so muscular people run higher and people with low muscle mass run lower creatinine, shifting the ratio. That is why a clinician reviews context before interpreting it.
What are typical BUN and creatinine levels?
Typical adult reference ranges are BUN 7–20 mg/dL (2.5–7.1 mmol/L) and creatinine 0.7–1.2 mg/dL (62–106 µmol/L). Reference ranges vary between laboratories, and creatinine norms also differ by sex and muscle mass, so your own lab report's printed range is always the authority. This calculator shows each value's position against typical ranges as context only.
If my ratio is normal, does that mean my kidneys are fine?
Not necessarily. A normal ratio (10–20) with both BUN and creatinine elevated can still indicate significant kidney involvement — in intrinsic kidney injury both markers rise together, so the ratio stays unremarkable while absolute values climb. A normal ratio only means the two markers are in their usual proportion; it does not rule out kidney disease. Any abnormal lab value deserves discussion with a clinician.
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