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BUN Creatinine Ratio Formula
The BUN/creatinine ratio compares blood urea nitrogen to creatinine after both values are expressed in the standard U.S. units used for the ratio: BUN in mg/dL and creatinine in mg/dL.
Ratio = BUN / Creatinine
BUN = Ratio * Creatinine
Creatinine = BUN / Ratio
Unit conversions used before calculating the ratio:
BUN_mg/dL = Urea_mmol/L * 2.801
BUN_mg/dL = Urea_mg/dL * 0.4665
Creatinine_mg/dL = Creatinine_µmol/L / 88.42
Creatinine_mg/dL = Creatinine_mg/L / 10
- Ratio is the BUN/creatinine ratio, usually written as a number followed by “:1”.
- BUN is blood urea nitrogen in mg/dL.
- Creatinine is serum creatinine in mg/dL.
- Urea may appear on some lab reports instead of BUN. It is converted to BUN before the ratio is calculated.
In “ratio from labs” mode, you enter BUN or urea and creatinine, and the calculator divides BUN by creatinine. In “missing value” mode, you enter the ratio plus one known lab value, and the calculator rearranges the same formula to estimate the missing BUN, urea, or creatinine value.
BUN/Creatinine Ratio Interpretation and Unit Conversions
Adult reference ranges can vary by lab, age, hydration status, diet, and clinical condition. The ranges below are general interpretation bands for checking a result.
| BUN/creatinine ratio | General interpretation | Common clinical considerations |
|---|---|---|
| Less than 10:1 | Low | May be seen with low urea production, liver disease, low protein intake, or some kidney-related patterns. |
| 10:1 to 20:1 | Typical adult range | Often considered within the usual adult reference range when the individual BUN and creatinine values are also appropriate. |
| Greater than 20:1 | High | May be associated with dehydration, reduced kidney blood flow, high protein intake, gastrointestinal bleeding, or catabolic states. |
| Lab report unit | Converted value used in ratio | Example |
|---|---|---|
| BUN mg/dL | No conversion | 14 mg/dL BUN = 14 mg/dL BUN |
| Urea mmol/L | Multiply by 2.801 | 5 mmol/L urea = 14.0 mg/dL BUN |
| Urea mg/dL | Multiply by 0.4665 | 30 mg/dL urea = 14.0 mg/dL BUN |
| Creatinine µmol/L | Divide by 88.42 | 80 µmol/L = 0.90 mg/dL creatinine |
Examples
Example 1: Calculate the ratio from BUN and creatinine
You have a BUN of 18 mg/dL and a creatinine of 1.0 mg/dL.
Ratio = 18 / 1.0 = 18
The BUN/creatinine ratio is 18:1, which falls within the usual adult range of about 10:1 to 20:1.
Example 2: Find missing BUN from creatinine and ratio
You have a creatinine of 0.9 mg/dL and want a BUN/creatinine ratio of 15:1.
BUN = 15 * 0.9 = 13.5
The estimated BUN is 13.5 mg/dL.
FAQ
What is a normal BUN/creatinine ratio?
A common adult reference range is about 10:1 to 20:1. A result in this range is often considered typical, but it should be interpreted with the actual BUN value, creatinine value, estimated GFR, hydration status, symptoms, medications, and the reference ranges on your lab report.
Why does the calculator convert urea to BUN?
The standard BUN/creatinine ratio uses BUN in mg/dL, not urea directly. Some lab reports list urea instead of BUN, especially outside the United States. The calculator converts urea to the equivalent BUN value first, then divides by creatinine.
Does a high BUN/creatinine ratio always mean kidney disease?
No. A high ratio can happen when BUN rises more than creatinine, such as with dehydration, reduced blood flow to the kidneys, high protein intake, gastrointestinal bleeding, or steroid use. Kidney disease is only one possible consideration. If your result is abnormal, compare it with the rest of your kidney panel and ask a clinician how it applies to your situation.
