What this calculator does
Urea and creatinine are both cleared by the kidneys, but they behave differently. Urea is partly reabsorbed when the body is conserving water, while creatinine is not, so the ratio between them shifts depending on why kidney markers have risen.
That difference is the whole point of the ratio. A value above about 20 to 1 suggests urea is being reabsorbed, which points toward reduced blood flow to the kidneys rather than damage within them.
The formula
Divide blood urea nitrogen by creatinine, both in mg/dL. The ratio is dimensionless and is conventionally expressed against 1.
| Term | Meaning |
|---|---|
| BUN | Blood urea nitrogen, a waste product of protein metabolism. |
| Creatinine | A waste product of muscle metabolism, filtered but barely reabsorbed. |
| Prerenal | Reduced blood flow reaching the kidney, classically raising the ratio. |
The inputs explained
| Field | What to enter |
|---|---|
| Blood urea nitrogen (BUN) (mg/dL) | Blood urea nitrogen in mg/dL. |
| Creatinine (mg/dL) | Serum creatinine in mg/dL, from the same sample. |
When to use it
Classifying a rise in kidney markers
The ratio helps separate reduced blood flow from damage within the kidney.
Assessing hydration status
Dehydration characteristically raises urea more than creatinine.
Reading a metabolic panel
Both values appear routinely, and the ratio between them adds information.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each urea level imply?
The same creatinine at three urea levels.
| Blood urea nitrogen | BUN-to-creatinine ratio | Reading |
|---|---|---|
| 10 mg/dL | 10.0:1 | Within the typical range |
| 20 mg/dL | 20.0:1 | Within the typical range |
| 40 mg/dL | 40.0:1 | Above the typical range: can suggest reduced blood flow to the kidneys, dehydration or GI bleeding |
Questions
What raises the ratio besides dehydration?
Gastrointestinal bleeding, a high protein intake, some medications and heart failure can all raise urea relative to creatinine. A raised ratio narrows the possibilities without identifying which applies.
What lowers it?
Liver disease, since urea is made in the liver, along with low protein intake, malnutrition and pregnancy. Low muscle mass can also lower creatinine, which raises the ratio for an unrelated reason.
Is BUN the same as urea?
Not numerically. BUN measures the nitrogen content of urea and is used in the United States; many other countries report urea itself, which is roughly 2.14 times the BUN figure. The ratio bands here assume BUN.
Can this diagnose kidney disease?
No. It is one pattern among several, interpreted alongside creatinine, eGFR, urine studies and the clinical picture. An abnormal ratio should be discussed with the clinician who ordered the test.
For estimated filtration rate, see the creatinine clearance calculator. For the liver enzyme pattern, see the AST-to-ALT ratio calculator.