BUN/Creatinine Ratio Calculator
What the BUN/creatinine ratio tells you
The BUN/creatinine (BUN/Cr) ratio compares two routine blood markers that are usually read together because the ratio can provide more context than either number alone:
- BUN (blood urea nitrogen): a measure of urea nitrogen that reflects how much of a protein by-product is circulating in the blood before the kidneys clear it.
- Creatinine: a waste product from normal muscle activity that the kidneys also remove from the bloodstream.
Because BUN can change with hydration, protein intake, bleeding in the digestive tract, medications, and other non-kidney influences, while creatinine is shaped by filtration as well as muscle mass, the ratio can sometimes help clinicians decide whether the pattern looks more like reduced blood flow to the kidneys (prerenal causes), intrinsic kidney problems, or non-kidney factors. It is still only one clue. Symptoms, urine results, medications, trend over time, and other labs often including eGFR matter just as much as the ratio itself.
BUN/creatinine ratio formula
For BUN and creatinine reported in the usual U.S. unit of mg/dL, this calculator uses a direct division: divide the BUN value by the creatinine value, and the units cancel out to give a unitless ratio.
Example: BUN 28 mg/dL and creatinine 1.1 mg/dL → ratio = 28 ÷ 1.1 ≈ 25.5.
Unit note: This calculator assumes both values are entered in mg/dL. If one result is in another unit system, convert it first; mixing units will not produce a meaningful BUN/creatinine ratio.
Typical BUN/creatinine ratio ranges and what they can suggest
BUN/creatinine ratio references often use about 10:1 to 20:1 for adults, but that range is only a rough guide. Laboratories, age, diet, hydration, and medical context can all shift what is considered expected, so the number should be read as a clue rather than a verdict.
| Ratio (BUN/Cr) | Common interpretation (not diagnostic) | Examples of contributing factors |
|---|---|---|
| ~10–20 | Often seen in many adults, but not automatically normal if the individual BUN and creatinine values are abnormal | Results still depend on hydration status, muscle mass, and the lab's reference interval |
| >20 | May point toward reduced kidney perfusion or increased urea production | Dehydration, vomiting/diarrhea, diuretics, heart failure, GI bleeding, high-protein intake, corticosteroids |
| <10 | May occur when urea production is low or creatinine is relatively higher | Low protein intake, severe liver disease, muscle injury, some intrinsic kidney disorders |
How to interpret a BUN/creatinine ratio responsibly
- Look at both numbers, not just the ratio: a BUN/creatinine ratio can land in the expected range even when BUN and creatinine are both abnormal, and it can look high when one value is driving the entire change.
- Consider recent hydration and intake: fluid loss can concentrate BUN more than creatinine, so recent vomiting, diarrhea, heavy sweating, or a short period of low intake may raise the ratio.
- Consider muscle mass and body composition: low muscle mass can lower creatinine and make the ratio appear higher, while very muscular people may have a higher baseline creatinine that pulls the ratio down.
- Compare with prior results: a single BUN/creatinine ratio is a snapshot, but a trend over time often tells a clearer story about what is changing and whether the pattern is persistent.
When the ratio looks unusual, clinicians usually check whether the direction of change matches the BUN value, the creatinine value, and the rest of the picture rather than treating the ratio as a stand-alone answer.
Worked example: BUN/creatinine ratio with real lab values
Here is a BUN/creatinine ratio example using the same division the calculator performs:
- BUN: 28 mg/dL
- Creatinine: 1.1 mg/dL
Calculation: 28 ÷ 1.1 = 25.5.
Interpretation: A ratio around 25.5 is above the common 10–20 range, so it may fit a pattern seen with dehydration, reduced blood flow to the kidneys, or another cause of higher BUN relative to creatinine. It does not identify the cause by itself. In practice, a clinician would compare it with the absolute BUN and creatinine values, recent changes in hydration, urine output, urinalysis, symptoms such as thirst or dizziness, and any other lab results before deciding what it means.
Limitations and assumptions for BUN/creatinine ratio results
- Not medical advice / not a diagnosis: This calculator provides an educational estimate only and cannot diagnose kidney disease, dehydration, liver disease, or any other condition.
- Units assumption: Inputs must both be in mg/dL, because the calculator divides the two numbers directly without attempting unit conversion.
- Lab variability: Reference ranges, assay methods, and reporting styles differ by laboratory, so the interval printed on your report should take priority over a generic rule of thumb.
- Population context: Interpretation can differ for children, pregnant patients, older adults, and people with very low or very high muscle mass, because creatinine and BUN do not behave the same way in every body.
- Confounders: Diet, gastrointestinal bleeding, corticosteroids, catabolic states, liver disease, muscle injury, and many medications can change BUN and/or creatinine independent of kidney filtration, which can shift the ratio in ways that look dramatic but are not specific.
- Ratio alone is incomplete: Clinicians usually interpret BUN/creatinine alongside the creatinine trend, eGFR, urine output, urinalysis, and the patient's symptoms and vital signs.
If you have concerning symptoms such as confusion, severe weakness, chest pain, fainting, very low urine output, or persistent vomiting or diarrhea, seek urgent medical care.
BUN/creatinine ratio FAQ
What is a normal BUN/creatinine ratio?
Many references use roughly 10 to 20 in adults, but no single cutoff fits every patient. Your lab's interval and the rest of your results matter more than the ratio alone, especially when BUN and creatinine are moving for different reasons.
What can raise the BUN/creatinine ratio?
Dehydration is a common reason because it can raise BUN more than creatinine. The ratio can also climb with reduced blood flow to the kidneys, gastrointestinal bleeding, a high-protein intake, corticosteroids, or other states that increase urea production.
What can lower the BUN/creatinine ratio?
A lower ratio may appear when the body makes less urea, such as in severe liver disease or low protein intake, or when creatinine is relatively higher because of muscle injury or another cause. The absolute BUN and creatinine values still need to be checked, because the ratio alone does not explain why the numbers moved.
Does dehydration affect the BUN/creatinine ratio?
Yes. When fluid losses concentrate the blood, BUN often rises faster than creatinine, which can push the ratio upward. That said, hydration is only one piece of the picture, and a high ratio still needs to be read with symptoms and the rest of the lab work.
Do the units matter?
Yes. This calculator assumes both inputs are in mg/dL. If your report lists a different unit system, convert both numbers before dividing so the ratio stays meaningful.
Arcade Mini-Game: BUN/Creatinine Ratio Input Check
Use this quick arcade run to practice spotting which lab values belong in a BUN/creatinine ratio calculation and which distracting assumptions should be left out before you trust the result.
Start the game, then use your pointer or arrow keys to catch useful BUN/creatinine inputs and avoid bad assumptions.
Status messages will appear here.