BUN:creatinine ratio = BUN (mg/dL) ÷ creatinine (mg/dL). About 10:1 is typical; above 20:1 suggests a prerenal cause such as dehydration or low kidney blood flow, and below 10:1 can reflect low protein intake, liver disease or muscle breakdown. The ratio is a clue, not a diagnosis: it is read together with the creatinine level, urine studies and the patient.
This calculator accepts US units (mg/dL) or SI units (urea in mmol/L, creatinine in µmol/L), converts between them, and uses the ranges in StatPearls and the NCBI Bookshelf's Clinical Methods. It is written for clinicians and students.
How to use this calculator
- Pick US or SI units to match your lab report.
- Enter BUN (or urea) and creatinine from the same blood sample.
- Read the ratio and the band. The calculator also shows the other unit system and the urea:creatinine ratio with both values in mmol/L.
- Compare with earlier results. A change from the patient's usual ratio says more than a single value.
How the ratio is calculated
- Ratio = BUN (mg/dL) ÷ creatinine (mg/dL).
- Urea (mmol/L) = BUN (mg/dL) × 0.357. Clinical Methods gives BUN 5–20 mg/dL as 1.8–7.1 mmol/L of urea.
- Creatinine (µmol/L) = creatinine (mg/dL) × 88.4. Clinical Methods gives 0.6–1.2 mg/dL as 53–106 µmol/L.
Why the ratio moves: creatinine is filtered by the glomerulus and is not reabsorbed, but a share of filtered urea is reabsorbed, and Clinical Methods notes more is reabsorbed when urine flow is low, in volume contraction and in heart failure. StatPearls explains that in prerenal disease urea is passively reabsorbed in the proximal tubule along with sodium and water. BUN therefore rises faster than creatinine and the ratio climbs.
Worked examples
Six typical scenarios run through the calculator's own function, with both values in mg/dL.
| BUN | Creatinine | Ratio | Band | Scenario |
|---|---|---|---|---|
| 15 mg/dL | 1 mg/dL | 15.0:1 | Typical range (10–20:1) | Healthy adult |
| 50 mg/dL | 5 mg/dL | 10.0:1 | Typical range (10–20:1) | Advanced kidney failure, no extra factor |
| 100 mg/dL | 5 mg/dL | 20.0:1 | Typical range (10–20:1) | Same creatinine, BUN doubled |
| 45 mg/dL | 1.5 mg/dL | 30.0:1 | High (above 20:1) | Dehydration or low perfusion |
| 30 mg/dL | 2 mg/dL | 15.0:1 | Typical range (10–20:1) | Tubular injury (ATN) pattern |
| 8 mg/dL | 1.2 mg/dL | 6.7:1 | Low (below 10:1) | Low protein intake or liver disease |
Rows 2 and 3 follow the example in Clinical Methods: with a creatinine of 5.0 mg/dL a BUN close to 50 is expected, and a BUN of 100 should start a search for factors outside the kidney. The text adds that the 10:1 rule applies best in moderate to advanced kidney failure.
Ratio for common BUN and creatinine values
Ratio for common BUN and creatinine pairs (mg/dL). Values above 20 are shown in bold.
| Creatinine ↓ / BUN → | 10 | 20 | 30 | 40 | 60 |
|---|---|---|---|---|---|
| 0.8 | 12.5 | 25.0 | 37.5 | 50.0 | 75.0 |
| 1 | 10.0 | 20.0 | 30.0 | 40.0 | 60.0 |
| 1.5 | 6.7 | 13.3 | 20.0 | 26.7 | 40.0 |
| 2 | 5.0 | 10.0 | 15.0 | 20.0 | 30.0 |
| 3 | 3.3 | 6.7 | 10.0 | 13.3 | 20.0 |
Working in SI units
Three sample results reported as urea (mmol/L) and creatinine (µmol/L), converted to US units and shown as both ratio conventions.
| Urea (mmol/L) | Creatinine (µmol/L) | BUN (mg/dL) | Creatinine (mg/dL) | BUN:Cr | Urea:Cr (mmol/L) |
|---|---|---|---|---|---|
| 5 | 80 | 14.0 | 0.90 | 15.5:1 | 62 |
| 14 | 150 | 39.2 | 1.70 | 23.1:1 | 93 |
| 25 | 400 | 70.0 | 4.52 | 15.5:1 | 62 |
In the UK and Europe the ratio is often quoted as urea:creatinine with both in mmol/L, which gives numbers about four times larger than the US BUN:creatinine ratio. Do not compare the two without converting.
Prerenal, intrinsic and postrenal patterns
The table below is general guidance drawn from StatPearls. Real patients often have more than one process, and pre-existing kidney disease blurs every pattern.
| Pattern | Typical BUN:Cr | Other clues (StatPearls) |
|---|---|---|
| Normal | About 10:1 | BUN and creatinine in range |
| Prerenal (low perfusion) | Above 20:1 | FENa below 1%, urine osmolality above 500, creatinine falls within 24–72 h of fluids |
| Intrinsic (ATN) | About 15:1; below 20:1 | FENa above 2%, urine osmolality below 450, creatinine rising 0.3–0.5 mg/dL per day |
| Postrenal (obstruction) | Variable; below 20:1 in the archived StatPearls table | Imaging for obstruction; low urine flow can increase urea reabsorption |
StatPearls calls fluid responsiveness the gold standard for identifying prerenal kidney injury: if creatinine returns towards baseline within 24 to 72 hours after the volume deficit is corrected, the cause was prerenal. The archived StatPearls azotemia chapter adds that there is little evidence the ratio alone reliably separates prerenal azotemia from ATN.
Common causes of a high ratio
- Low kidney perfusion: dehydration, vomiting or diarrhea, bleeding, over-diuresis, heart failure or cirrhosis (StatPearls).
- Extra urea production: upper GI bleeding, fever, steroids and high protein intake raise BUN but not creatinine (Clinical Methods).
- Low muscle mass: frail or older people make less creatinine, so the ratio can look high with ordinary BUN values.
Common causes of a low ratio
- Low protein intake or malnutrition.
- Advanced liver disease, which reduces urea production.
- Rhabdomyolysis, which releases creatinine from damaged muscle.
- Dialysis, which removes urea more effectively than creatinine.
All four come from Clinical Methods, which also lists extra urea excretion as in sickle cell anemia.
Common mistakes
- Mixing units. Entering urea in mmol/L with creatinine in mg/dL gives a meaningless number. Pick one system.
- Treating BUN as whole urea. Urea (mmol/L) and BUN (mg/dL) differ by a factor of about 2.8, so use the conversion.
- Reading the ratio alone. A ratio of 25 with a creatinine of 0.7 is a different situation from 25 with a creatinine of 4. Look at the absolute values first.
- Ignoring the lab method. Clinical Methods notes the creatinine range depends on the assay method (enzymatic versus Jaffé).
- Stopping at one test. If there is an acid-base problem, work it up with the anion gap calculator and Winter's formula. In hyperglycemia use the corrected sodium calculator. The A/G ratio calculator covers the protein part of the same metabolic panel.
For healthcare professionals and students only. The BUN:creatinine ratio is one clue in assessing kidney function and volume status, and this tool offers no medical advice. Interpret it with creatinine trends, urine studies, the history and your laboratory's own ranges. Patients should ask their own doctor to explain their results.
Frequently asked questions
What does the BUN/creatinine ratio mean?
It compares two waste products the kidneys clear. Creatinine is filtered and not reabsorbed, while urea is partly reabsorbed, more so when kidney blood flow or urine flow is low. A ratio well above the usual 10:1 suggests something is raising urea beyond what kidney function alone explains, such as dehydration, low perfusion or a GI bleed.
How do you calculate the BUN creatinine ratio?
Divide BUN in mg/dL by creatinine in mg/dL. For BUN 45 and creatinine 1.5, the ratio is 45 ÷ 1.5 = 30.0:1.
What is a normal BUN/creatinine ratio?
Clinical Methods uses about 10:1 as the expected ratio when nothing outside the kidneys is affecting urea, and StatPearls also calls a ratio of about 10 typical. This calculator treats 10–20:1 as the usual range, but labs report their own range and the ratio has no universally agreed cut-off.
What does a high BUN/creatinine ratio mean?
StatPearls puts the prerenal pattern above 20:1: dehydration, bleeding, heart failure or other causes of low kidney perfusion, where urea is reabsorbed along with sodium and water. Clinical Methods adds that creatinine, unlike urea, is largely unaffected by GI bleeding, fever and steroids, so these also raise the ratio.
What does a low BUN/creatinine ratio mean?
Clinical Methods lists low protein intake, reduced urea production in advanced liver disease, extra urea loss (as in sickle cell anemia), increased creatinine production in rhabdomyolysis, and dialysis, which clears urea more than creatinine.
What BUN:creatinine ratio suggests prerenal azotemia?
Above 20:1 in StatPearls. It also gives about 15:1 for acute tubular necrosis. Treat both as rough guides: the archived StatPearls azotemia chapter notes there is little evidence that the ratio reliably separates prerenal azotemia from ATN.
How do you calculate the ratio with SI units (mmol/L and µmol/L)?
Convert first: BUN (mg/dL) = urea (mmol/L) ÷ 0.357, and creatinine (mg/dL) = creatinine (µmol/L) ÷ 88.4. Urea 14 mmol/L with creatinine 150 µmol/L is BUN 39.2 mg/dL and creatinine 1.70 mg/dL, a ratio of 23.1:1.
Is BUN the same as urea?
No. US labs report blood urea nitrogen, only the nitrogen part of the urea molecule; European labs usually report whole urea in mmol/L. Clinical Methods gives the BUN range 5–20 mg/dL as 1.8–7.1 mmol/L of urea, a factor of about 0.357.
What is the urea to creatinine ratio in mmol/L?
If both are in mmol/L (creatinine µmol/L ÷ 1000), multiply the BUN:creatinine ratio by about 4.04. A BUN:creatinine ratio of 20:1 is a urea:creatinine ratio of about 81. This is simple unit arithmetic, so check which convention your lab uses before comparing.
Can a high BUN/creatinine ratio be normal?
Yes. Clinical Methods notes a person eating a lot of protein can have a BUN of 20 mg/dL, and BUN varies with hydration and protein breakdown. A single high ratio with normal creatinine is often not a kidney problem. Ask the clinician who ordered the test to interpret it.
Sources & method
- Clinical Methods, 3rd ed. (NCBI Bookshelf) — BUN and creatinine (normal ranges in mg/dL and SI, 10:1 ratio, causes of a low ratio)
- StatPearls (NCBI Bookshelf) — Prerenal kidney failure (ratio about 10 normally, ~15:1 in ATN, above 20:1 prerenal; FENa)
- StatPearls (NCBI Bookshelf) — Azotemia (archived; prerenal, intrinsic and postrenal patterns)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.