# What is my BUN creatinine ratio?

Divides blood urea nitrogen (BUN) by serum creatinine, or urea by creatinine in SI units, and shows whether the ratio is in the usual 10 to 20 range.

- Page: https://www.acalculator.org/health/bun-creatinine-ratio-calculator
- JSON spec: https://www.acalculator.org/health/bun-creatinine-ratio-calculator.json
- Version: 619aeaefd2d2

## Default answer

Example with the default inputs (Units US (mg/dL), BUN (mg/dL) 18, Creatinine (mg/dL) 1): The BUN to creatinine ratio is 18: Usual range (10 to 20).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| units | Units | US units (BUN and creatinine in mg/dL) or SI units (urea in mmol/L, creatinine in µmol/L). |
| bun | BUN (mg/dL) | Blood urea nitrogen, in mg/dL. |
| cr | Creatinine (mg/dL) | Serum creatinine, in mg/dL. |
| urea | Urea (mmol/L) | Serum urea, in mmol/L. |
| crSi | Creatinine (µmol/L) | Serum creatinine, in µmol/L. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| ratio | BUN:creatinine ratio | BUN (mg/dL) divided by creatinine (mg/dL), to 1 decimal. |
| range | Range | Whether the ratio is low, in the usual range, or high. |
| siRatio | Urea:creatinine ratio (SI) | Urea (mmol/L) divided by creatinine (mmol/L), the SI form of the ratio, to a whole number. |

## Method

Ratio = BUN (mg/dL) ÷ creatinine (mg/dL). SI: BUN = urea (mmol/L) × 2.8014; creatinine (mg/dL) = µmol/L ÷ 88.4; SI ratio = urea (mmol/L) ÷ creatinine (mmol/L).

## Assumptions

- BUN and creatinine from the same blood sample.
- The ratio is rounded to 1 decimal and read from the rounded value: under 10 low, 10 to 20 usual, over 20 high.
- The ratio alone does not show the cause; it is read with the separate BUN and creatinine values.

## Worked examples

1. units = us, bun = 18, cr = 1 gives ratio = 18, range = Usual range (10 to 20), siRatio = 73.
2. units = us, bun = 40, cr = 1.2 gives ratio = 33.3, range = High (over 20). Source: 40 ÷ 1.2 = 33.33, over 20 (Hosten 1990).
3. units = si, urea = 5, crSi = 176.8 gives ratio = 7, range = Low (under 10), siRatio = 28.
4. units = us, bun = 20, cr = 1 gives ratio = 20, range = Usual range (10 to 20).

## FAQ

### What is the BUN creatinine ratio?

It is the blood urea nitrogen (BUN) divided by the serum creatinine, both in mg/dL. Both are waste products that the kidneys clear, but the body handles them differently, so their ratio can hint at why kidney tests are abnormal.

### What is a normal BUN creatinine ratio?

The ratio is usually about 10 to 20. Labs set their own reference range, so check the one on your report.

### What does a high BUN creatinine ratio mean?

A ratio over 20 is often seen when less blood reaches the kidneys (for example dehydration or heart failure), with bleeding in the stomach or upper gut, with a high protein intake, or with steroid treatment. It is a clue, not a diagnosis.

### What does a low BUN creatinine ratio mean?

A ratio under 10 can go with a low protein intake, severe liver disease (the liver makes urea), or muscle breakdown that raises creatinine. It can also be normal in some people.

### How do I convert urea in mmol/L to BUN?

Multiply urea in mmol/L by 2.8014 to get BUN in mg/dL, because each urea molecule holds two nitrogen atoms (2 × 14.007 g/mol). Divide creatinine in µmol/L by 88.4 to get mg/dL. The page does this for you.

## Sources

- Hosten AO. BUN and creatinine. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; 1990. Chapter 193. https://www.ncbi.nlm.nih.gov/books/NBK305/
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150. https://kdigo.org/guidelines/ckd-evaluation-and-management/ (1 mg/dL creatinine = 88.4 µmol/L)
- IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard atomic weights: nitrogen 14.007. https://ciaaw.org/nitrogen.htm