BUN-to-Creatinine Ratio Calculator

About this calculator

The BUN-to-creatinine ratio divides blood urea nitrogen by serum creatinine to help distinguish between causes of kidney dysfunction: in conventional US units (mg/dL), a normal ratio runs roughly 10:1 to 20:1; in SI units (mmol/L for both, or BUN converted to urea), the reference range and the math differ, so it matters which unit system a lab result is reported in. Both markers rise when kidney function drops, but they don’t always rise at the same rate, and that gap is diagnostically useful.

A ratio elevated above the normal range (often greater than 20:1) points toward a prerenal cause — dehydration, heart failure, or reduced blood flow to the kidneys — where the kidney tissue itself is intact but under-perfused, causing urea to be reabsorbed disproportionately more than creatinine. A ratio within the normal range but with both values elevated suggests intrinsic kidney damage instead, while a low ratio can point toward liver disease, malnutrition, or a low-protein diet. Physicians use it alongside a full metabolic panel, not in isolation, to help narrow the differential for acute kidney injury.

This calculator divides your BUN by your creatinine value in either conventional or SI units and flags whether the result suggests a prerenal pattern.