BUN-to-Creatinine Ratio Calculator

What Are BUN and Creatinine?

Blood Urea Nitrogen (BUN) measures the amount of nitrogen in the blood that comes from urea — a waste product formed in the liver when protein is broken down. Urea travels through the bloodstream to the kidneys, where it is filtered and excreted in urine. Elevated BUN can indicate poor kidney filtration, dehydration, high protein intake, or gastrointestinal bleeding.

Creatinine is a waste product produced at a relatively constant rate from the normal breakdown of creatine phosphate in muscle tissue. Because production is constant, blood creatinine levels are a reliable indicator of how well the kidneys are filtering: if filtration drops, creatinine accumulates. Creatinine is used to calculate eGFR (estimated Glomerular Filtration Rate), the standard measure of kidney function.

The BUN/Creatinine Ratio

The ratio helps differentiate why BUN and creatinine are elevated:

Normal ratio: 10 to 20 — Both markers are within normal range.

Ratio below 10: This suggests the BUN is disproportionately low relative to creatinine. Causes include liver disease (reduced urea synthesis), malnutrition (low protein intake), or dialysis patients.

Ratio 20 to 40 (elevated): Suggests pre-renal causes — conditions that reduce blood flow to the kidneys before they can filter it. Most common cause: dehydration. Others include congestive heart failure, excessive protein intake, or gastrointestinal bleeding (digested blood raises BUN significantly).

Ratio above 40 (severely elevated): Strongly suggests pre-renal azotemia — severely reduced kidney perfusion. This is a medical emergency requiring prompt fluid resuscitation. Common causes include severe dehydration, hemorrhagic shock, or complete urinary obstruction.

Differentiating Pre-Renal, Renal, and Post-Renal

• Pre-renal: Problem is before the kidney (reduced blood flow). BUN/Cr > 20. Kidneys are structurally normal but under-perfused. Corrects rapidly with fluids.
• Renal (intrinsic): The kidney itself is damaged (acute kidney injury, glomerulonephritis, acute tubular necrosis). BUN/Cr typically 10–15. Both markers rise proportionally.
• Post-renal (obstructive): Blockage prevents urine from leaving (kidney stones, enlarged prostate, tumor). BUN/Cr variable, typically elevated.

Unit Conversion (SI)

In SI units: BUN (mmol/L) × 2.8 = BUN (mg/dL). Creatinine (µmol/L) / 88.4 = Creatinine (mg/dL). The calculator converts automatically.

This calculator is for educational purposes only. Lab results must be interpreted by a healthcare provider in the context of the complete clinical picture.