BUN to Creatinine Ratio Calculator
Divide BUN by creatinine to see the 10 to 20 ratio and whether the pattern leans pre-renal.
What it does
BUN and creatinine are two of the oldest kidney labs in medicine. BUN measures the nitrogen part of urea, a waste product of protein breakdown. Creatinine is a breakdown product of muscle creatine. The kidneys clear both, but not in the same way. Urea is filtered and partly reabsorbed, while creatinine is filtered and only slightly secreted. That difference is why the ratio of one to the other can point toward a pattern.
The calculation is a simple division: BUN in mg/dL divided by creatinine in mg/dL. A result near 10 to 20 is the usual range. A ratio above 20 is often called pre-renal, meaning the kidneys are seeing less blood flow and reabsorbing more urea. A ratio near 10 or below can appear with intrinsic kidney disease, low protein intake, or advanced liver disease.
The ratio is easy to misuse. Dehydration raises it, but so does a GI bleed, a high protein diet, steroid treatment, or a catabolic state. A low muscle mass can push creatinine down and the ratio up without any kidney problem at all. A very muscular person can do the opposite. The number is a prompt to look at the patient, not a standalone answer.
Units cause real errors here. US labs report BUN in mg/dL. Many labs outside the US report urea in mmol/L, which is the whole molecule, not the nitrogen fraction. Urea in mmol/L is about 2.8 times the BUN value in mg/dL, but only because of the molecular weight and the nitrogen content. Dividing urea mmol/L by creatinine mg/dL gives a meaningless number. This page converts first.
Use the ratio together with volume status, medications, urine output, and the rest of the metabolic panel. A high ratio with a normal creatinine can simply mean a low protein intake or a bit of dehydration. A normal ratio does not rule out kidney disease, especially when both values are abnormal. The trend over time is often more useful than any single ratio.
How to use
- Choose US units if your lab reports BUN and creatinine in mg/dL, or metric if it reports urea in mmol/L and creatinine in umol/L.
- Enter the BUN value, or the urea value if you chose metric. Do not enter urea as if it were BUN.
- Enter serum creatinine from the same draw.
- Read the ratio and the band. The band uses 10 and 20 as the usual boundaries.
- Check the note for the pattern the ratio can fit, then review volume status, medications, and urine output.
- Repeat the calculation after treatment or hydration to see whether the ratio moves.
Formula
- BUN
- Blood urea nitrogen in mg/dL.
- SCr
- Serum creatinine in mg/dL.
- 2.8
- Approximate factor from urea mmol/L to BUN mg/dL.
- 88.4
- Factor from creatinine umol/L to mg/dL.
- Ratio
- BUN divided by creatinine, with units canceled.
The ratio is unitless because both inputs end up in mg/dL. BUN is not the same as urea: BUN counts only the nitrogen, while urea includes the carbon, oxygen, and hydrogen. In SI, urea mmol/L multiplied by 2.8 gives BUN mg/dL, and creatinine umol/L divided by 88.4 gives mg/dL. The 10 to 20 range is a convention, not a sharp cutoff. Small changes in either value can move the ratio, and labs may use different assays.
Worked example
Example: BUN 28 mg/dL and creatinine 1.2 mg/dL
A patient has a BUN of 28 mg/dL and a serum creatinine of 1.2 mg/dL on the same panel.
- Divide BUN by creatinine: 28 / 1.2 = 23.33.
- Round to one decimal: 23.3.
- The ratio is above 20, so the band is High.
- Metric mirror: urea 10.0 mmol/L x 2.8 = 28 mg/dL.
- Creatinine 106 umol/L / 88.4 = 1.199 mg/dL.
- Divide again: 28 / 1.1991 = 23.35, which the display rounds to 23.4. The 0.1 gap from the US figure comes from rounding the SI conversion.
- The pattern can fit pre-renal physiology, but confirm with volume status, urine sodium, and the clinical picture.
Reference table
| BUN | Creatinine | Ratio | Band |
|---|---|---|---|
| 10 mg/dL (3.6 mmol/L urea) | 1.0 mg/dL (88 umol/L) | 10.0 | Normal range |
| 15 mg/dL (5.4 mmol/L urea) | 1.0 mg/dL (88 umol/L) | 15.0 | Normal range |
| 28 mg/dL (10.0 mmol/L urea) | 1.2 mg/dL (106 umol/L) | 23.3 US, 23.4 SI | High |
| 40 mg/dL (14.3 mmol/L urea) | 1.1 mg/dL (97 umol/L) | 36.4 | High |
| 10 mg/dL (3.6 mmol/L urea) | 2.0 mg/dL (177 umol/L) | 5.0 | Low |
Limits
- The ratio cannot separate dehydration from GI bleeding, steroid use, or a high protein diet.
- Low muscle mass lowers creatinine and raises the ratio without kidney disease.
- Liver disease and a low protein diet can lower BUN and pull the ratio down.
- Diuretics, fluid status, and recent contrast can change both values quickly.
Common mistakes
- Dividing BUN mg/dL by creatinine umol/L without converting.
- Using urea mmol/L as if it were BUN mg/dL.
- Calling a ratio above 20 proof of pre-renal disease.
- Ignoring creatine supplements, muscle mass, or amputation when reading creatinine.
- Rounding both values heavily before dividing, which can move the ratio by a full point.
Try it yourself
Quick checks. Answers sit tucked under each question.
BUN 15 mg/dL and creatinine 1.0 mg/dL. What is the ratio?
15 / 1.0 = 15.0, inside the usual 10 to 20 range.
BUN 40 mg/dL and creatinine 1.1 mg/dL. What is the ratio?
40 / 1.1 = 36.4, above 20. That pattern can appear with upper GI bleeding or dehydration.
Urea 7.0 mmol/L and creatinine 88 umol/L. What is the ratio?
BUN = 7.0 x 2.8 = 19.6 mg/dL. Creatinine = 88 / 88.4 = 0.995 mg/dL. Ratio = 19.6 / 0.995 = 19.7, near the top of the normal range.
Questions
What is BUN?
Blood urea nitrogen is the nitrogen portion of urea in the blood. It rises when the kidneys clear less urea or when the body makes more.
What does the ratio mean?
It compares urea nitrogen to creatinine. A high ratio suggests more urea reabsorption or more urea production relative to creatinine.
Why is the ratio high in GI bleeding?
Blood in the gut is digested into protein, which raises urea production. Creatinine does not rise in step, so the ratio climbs.
Can dehydration cause a high ratio?
Yes. Low blood volume makes the kidneys reabsorb more urea, while creatinine is less affected.
Why does my lab report urea instead of BUN?
Many countries report whole urea in mmol/L. Multiply by 2.8 to approximate BUN in mg/dL before comparing with US ranges.
Does a normal ratio rule out kidney disease?
No. Both values can be abnormal and still give a normal ratio. Use eGFR, urinalysis, and the clinical picture.
Sources
- NCBI Bookshelf: Renal Function Tests, StatPearls
- NCBI Bookshelf: BUN and Creatinine, Clinical Methods