BUN/Creatinine Ratio Calculator
The BUN/creatinine ratio divides your blood urea nitrogen by your serum creatinine, both reduced to mg/dL. Doctors use it to help tell where a rise in kidney markers is coming from: dehydration or reduced blood flow (prerenal), the kidney tissue itself (intrinsic), or something after the kidney. Enter values in U.S. units (mg/dL) or international units (urea mmol/L, creatinine umol/L) and the tool converts for you.
Formula
Worked example
With a BUN of 28 mg/dL and a serum creatinine of 1.0 mg/dL, the ratio is 28 ÷ 1.0 = 28. That is above the typical 10 to 20 band, a prerenal pattern often seen with dehydration or reduced kidney blood flow. If instead your report showed urea 10 mmol/L and creatinine 88 umol/L, that converts to BUN 28 mg/dL and creatinine 1.0 mg/dL, giving the same ratio of 28.
What the BUN/creatinine ratio measures
Blood urea nitrogen (BUN) reflects urea, a waste product of protein breakdown that the kidneys filter out. Serum creatinine reflects a waste product of muscle metabolism that is also filtered. Both rise when kidney function falls, but they respond differently to other factors, so the ratio between them carries information that neither value gives alone. Dividing BUN by creatinine, both in mg/dL, yields a unitless number that usually sits between about 10 and 20 in healthy adults.
Urea vs BUN, and international units
There is a common point of confusion: BUN measures only the nitrogen part of the urea molecule, while many labs outside the United States report whole urea, usually in mmol/L. Urea is about 2.14 times larger than BUN by weight, so the same blood sample produces very different looking numbers depending on which is reported. This calculator handles the conversion for you. Choosing urea in mmol/L multiplies by 2.8 to get BUN in mg/dL, choosing urea in mg/dL multiplies by 0.4667, and creatinine in umol/L is divided by 88.42 to reach mg/dL. The ratio itself is always computed from BUN and creatinine in mg/dL so the 10 to 20 band still applies.
Why the ratio shifts up or down
A high ratio (often above 20) tends to point to causes outside the kidney tissue itself. Dehydration, heart failure, or anything that slows blood flow to the kidneys lets more urea get reabsorbed while creatinine stays relatively steady, a pattern called prerenal. Gastrointestinal bleeding can also push it up because digested blood adds nitrogen, and a urinary tract obstruction (postrenal) can raise it too. A low ratio (below 10) can follow a low protein diet, liver disease that limits urea production, overhydration, or intrinsic kidney damage such as acute tubular necrosis that raises creatinine. The ratio narrows down the likely category but never names a single cause.
Reading your result safely
This calculator gives an estimate for general education and is not a diagnosis. Reference ranges vary slightly between laboratories, and the ratio is only meaningful when read alongside the actual BUN and creatinine numbers, your estimated glomerular filtration rate (eGFR), medications, and symptoms. A clinician interprets all of these together. If your values are outside your lab reference range or you feel unwell, contact a healthcare professional rather than acting on the ratio alone.
Interpreting the BUN/creatinine ratio
| Ratio | Band | Likely category | What it can suggest |
|---|---|---|---|
| Below 10 | Low | Intrinsic / other | Low protein intake, liver disease, overhydration, or intrinsic kidney damage (e.g. acute tubular necrosis) |
| 10 to 20 | Normal | Balanced | Typical balance between urea and creatinine |
| Above 20 | High | Prerenal / postrenal | Dehydration, reduced blood flow to the kidneys, GI bleeding, or an obstruction downstream |
General adult guide. A ratio outside the normal band is a clue, not a diagnosis. It is always read alongside the actual BUN and creatinine values, eGFR, and your symptoms.
Frequently asked questions
What is a normal BUN/creatinine ratio?
In adults, a ratio of roughly 10 to 20 is generally considered normal, though exact cutoffs vary slightly by laboratory. Values are most useful when interpreted with the underlying BUN and creatinine numbers and your overall clinical picture.
Does a high ratio always mean kidney disease?
No. A high ratio most often reflects a prerenal cause such as dehydration or reduced blood flow to the kidneys rather than damage to the kidney tissue itself. Gastrointestinal bleeding, a high protein intake, and downstream obstruction can also raise it. Only a clinician can determine the cause.
My lab reports urea in mmol/L, not BUN. Can I still use this?
Yes. Set the first selector to urea in mmol/L and the calculator converts it to BUN in mg/dL by multiplying by 2.8 before computing the ratio. You can also enter urea in mg/dL or creatinine in umol/L, and the converted mg/dL values are shown alongside the result so you can check the math.
What is the difference between urea and BUN?
BUN measures only the nitrogen atoms within the urea molecule, while urea is the whole compound. Urea is about 2.14 times larger than BUN by weight, so urea 10 mmol/L corresponds to a BUN of about 28 mg/dL. The ratio uses BUN in mg/dL, which is why the tool converts urea for you.