BUN Creatinine Ratio Calculator

What Is the BUN Creatinine Ratio?

The BUN/creatinine ratio—also written BUN:Cr, BUN/Cr, B/C ratio, or blood urea nitrogen-to-creatinine ratio—compares two serum measurements commonly included in a metabolic panel. BUN measures the nitrogen portion of urea formed during protein metabolism. Creatinine is generated largely from muscle creatine metabolism and is filtered by the kidneys.

The markers do not behave identically. Urea can be reabsorbed in the kidney tubules and is affected by protein intake, catabolism, gastrointestinal bleeding, liver urea production, and hydration or perfusion. Creatinine is affected by kidney filtration, muscle mass, diet, medications, and changing kidney function. Their relationship can therefore add context, but it cannot replace either source value.

For a filtration estimate based on age, sex, and serum creatinine, use the GFR Calculator. eGFR and the BUN:creatinine ratio answer different clinical questions.

BUN Creatinine Ratio Calculation Formula

If a report already gives BUN and creatinine in mg/dL, the calculation is direct. A BUN of 20 mg/dL and creatinine of 1.0 mg/dL gives 20 ÷ 1.0 = 20, reported as 20:1.

Do not divide urea in mmol/L directly by creatinine in µmol/L. They are different analytes and scales. SI mode converts the inputs first:

How to Calculate the BUN Creatinine Ratio

1. 1

   Use serum results from the same draw

   Do not mix a current BUN with an older creatinine or combine serum and urine measurements.

2. 2

   Identify the reported analyte

   Confirm whether the report says BUN in mg/dL or urea in mmol/L. Those inputs are not numerically interchangeable.

3. 3

   Choose the matching unit mode

   Use U.S. mode for BUN and creatinine in mg/dL. Use SI mode for urea in mmol/L and creatinine in µmol/L.

4. 4

   Convert SI values when necessary

   Convert urea to BUN and creatinine to mg/dL before calculating the conventional ratio.

5. 5

   Divide BUN by creatinine

   The calculator keeps full precision during conversion and displays the ratio rounded to one decimal place.

6. 6

   Interpret the complete pattern

   Compare with your laboratory's reference interval, separate values, eGFR, urine output, medications, symptoms, and prior results.

BUN Creatinine Ratio Calculation Examples

Normal BUN Creatinine Ratio Range

Reference intervals vary by laboratory, assay, population, and reporting convention. Some reports use a range extending above 20. The calculator uses 10–20 only as a familiar educational comparison and does not overwrite the interval printed beside your result.

Children often have lower creatinine because of lower muscle mass, so adult ratio boundaries should not be applied automatically. Pregnancy, advanced age, frailty, amputation, very high or low muscle mass, and critical illness can also change interpretation.

What Does a High BUN Creatinine Ratio Mean?

A ratio above approximately 20:1 means BUN is higher relative to creatinine. In a patient with azotemia, this pattern may support a prerenal process because reduced kidney perfusion can increase tubular urea reabsorption more than creatinine. Dehydration or volume depletion is one possible setting, but it is not the only one.

• Reduced kidney perfusion: volume depletion, low blood pressure, or decreased effective circulating volume can raise the ratio.
• Heart failure or cirrhosis: total body fluid may be high while effective kidney perfusion remains reduced.
• Gastrointestinal bleeding: digestion and absorption of blood protein can increase urea production.
• Protein and catabolism: high protein intake, corticosteroids, tissue breakdown, fever, or severe illness may raise BUN disproportionately.
• Low creatinine denominator: low muscle mass can make the ratio appear high even when BUN is not markedly elevated.

Merck's comparison of prerenal azotemia and acute tubular necrosis lists BUN:creatinine above 20:1 as one prerenal pattern, but it also emphasizes other findings and caveats. No single cutoff proves dehydration or determines treatment.

What Does a Low BUN Creatinine Ratio Mean?

A ratio below approximately 10:1 means BUN is low relative to creatinine. The pattern can result from reduced urea generation, greater creatinine generation, dilution, or some intrinsic kidney processes. It should not automatically be labeled kidney damage.

• Lower urea production: low protein intake, malnutrition, or impaired liver urea synthesis can lower BUN.
• Fluid excess: dilution and altered urea handling can contribute to a low result.
• Higher creatinine production: greater muscle mass, strenuous exercise, muscle injury, or creatine-related factors may increase the denominator.
• Intrinsic renal context: acute tubular injury can show a lower ratio than prerenal azotemia, but the ratio alone is not sufficiently specific.
• Medication and assay effects: some drugs change creatinine secretion or measurement without a matching change in BUN.

Why the Absolute BUN and Creatinine Values Matter

The related Serum Osmolality Calculator uses BUN as one solute in a concentration estimate. It does not interpret the BUN:creatinine relationship.

BUN Creatinine Ratio vs Other Kidney Ratios

This calculator is also not a creatinine-clearance formula and does not use age, sex, body weight, or a timed urine collection. Mixing these intents would produce a medically different tool.

Important Interpretation Limits

• Not diagnostic: the result cannot prove prerenal, intrinsic, or postrenal kidney injury.
• Not a hydration prescription: a high ratio does not automatically mean drinking more fluid is safe, especially with heart, kidney, or liver disease.
• Not pediatric-specific: children need age-appropriate reference values and clinical assessment.
• Not a trend: one pair of values cannot show whether kidney function is stable, improving, or worsening.
• Not valid across mismatched units: raw SI numbers cannot be divided directly to recreate the conventional BUN:Cr ratio.
• Not a substitute for eGFR: the ratio does not quantify glomerular filtration.

If hypernatremia is being evaluated, the Free Water Deficit Calculator answers a separate adult water-balance question and should not be inferred from this ratio.

Common BUN Creatinine Ratio Calculation Errors

• Entering urea as BUN: urea mmol/L requires conversion and is not a direct BUN value.
• Mixing mg/dL and µmol/L: creatinine must be converted before conventional division.
• Using urine measurements: this page calculates a serum ratio only.
• Ignoring the source values: a ratio can look normal even when both BUN and creatinine are abnormal.
• Calling 20 a universal cutoff: the laboratory's own interval and clinical context take priority.
• Assuming high means dehydration: perfusion, bleeding, protein metabolism, medication, and low muscle mass can also affect the result.

When to Seek Medical Help

Contact a clinician promptly for a new or rapidly changing kidney result, markedly reduced urine output, persistent vomiting or diarrhea, inability to drink, swelling, shortness of breath, confusion, or concern about medication effects. Black tarry stool or vomiting blood can indicate gastrointestinal bleeding and needs urgent assessment.