This calculator divides blood urea nitrogen (BUN) by serum creatinine to give the BUN/creatinine ratio, a unitless number that helps separate kidney problems from causes that sit before the kidney. A normal ratio is about 10:1 to 20:1.
What the BUN/Creatinine Ratio Is
Two waste products are measured here. BUN (blood urea nitrogen) reflects urea, a by-product of protein breakdown that the liver makes and the kidneys clear. Creatinine is a by-product of muscle metabolism cleared mostly by filtration. Both rise when kidney filtration falls — but they respond differently to blood flow and hydration, and that difference is what the ratio exposes. The formula is simply:
BUN/creatinine ratio = BUN (mg/dL) ÷ serum creatinine (mg/dL)
Because both inputs share the mg/dL unit, the result has no units. Use US conventional units for both: if a value is in SI units, convert creatinine with the unit converter first so the inputs match.
Worked Example
A patient has a BUN of 28 mg/dL and a serum creatinine of 1.0 mg/dL. Divide: 28 ÷ 1.0 = 28:1. That ratio sits well above the normal ceiling of 20:1, pointing toward a pre-renal picture such as dehydration rather than intrinsic kidney damage. Contrast that with a BUN of 14 mg/dL and a creatinine of 2.0 mg/dL: 14 ÷ 2.0 = 7:1, a low ratio more in keeping with liver disease or low protein intake despite the raised creatinine.
How to Read the Ratio
| Ratio | Category | Common causes |
|---|---|---|
| > 20:1 | High | Dehydration, reduced kidney blood flow, GI bleeding, high-protein intake, steroids |
| 10:1 – 20:1 | Normal | Typical reference range |
| < 10:1 | Low | Liver disease, low-protein diet, overhydration, pregnancy, malnutrition |
A high ratio most often signals a pre-renal state: when blood flow to the kidney drops, the tubules reabsorb more urea, so BUN climbs faster than creatinine. A low ratio suggests less urea is being produced or it is being diluted. A ratio inside the normal band with a raised creatinine points instead toward intrinsic kidney disease, where both markers rise together.
Clinical Uses
Clinicians use the ratio as a quick orientation when creatinine is rising: it helps distinguish pre-renal causes (volume depletion, heart failure, blood loss) from intrinsic kidney injury, and it can flag a gastrointestinal bleed when BUN jumps without a matching creatinine rise. It is a pointer, not a diagnosis — always read alongside the trend in creatinine, the clinical picture, and an estimate of overall kidney function from the creatinine clearance calculator.
What the Two Markers Actually Measure
Urea and creatinine reach the blood by different routes, and that is what makes their ratio informative. Urea is the liver’s end-product of protein breakdown; it is filtered by the kidney but also partly reabsorbed, and reabsorption rises when the body is conserving water. Creatinine is a steady by-product of muscle and is filtered with very little reabsorption, so it tracks filtration more cleanly. When the two are read together, the gap between a reabsorbed marker (urea) and a non-reabsorbed one (creatinine) reveals how hard the kidney is working to hold on to water — which is exactly the information the ratio condenses into a single number.
Pre-renal, Renal, and Post-renal Patterns
Acute kidney injury is traditionally grouped into three sites of trouble, and the BUN/creatinine ratio helps point to which one. In a pre-renal pattern — reduced blood flow from dehydration, heart failure, or blood loss — the kidney works hard to conserve water and reabsorbs urea along with it, so BUN climbs out of proportion to creatinine and the ratio rises above 20:1. In an intrinsic renal pattern, the filtering tissue itself is damaged, so both markers rise together and the ratio stays in the normal 10:1 to 20:1 band even as creatinine climbs. A post-renal pattern from obstruction can look pre-renal early (a high ratio) before settling toward the intrinsic pattern if the blockage persists. The ratio narrows the differential; it does not close it.
What Moves BUN and Creatinine Independently
Reading the ratio well means knowing what nudges each number on its own. BUN rises with a high-protein diet, gastrointestinal bleeding (digested blood is a protein load), corticosteroids, and any state that slows urine flow through the tubules. BUN falls with liver disease, because the liver makes less urea, and with a low-protein diet, overhydration, or pregnancy. Creatinine rises with reduced filtration but also reflects muscle mass — so a heavily muscled athlete runs a higher baseline creatinine, and a frail elderly patient a lower one, regardless of kidney function. Because the ratio is a quotient, anything that shifts one number without the other swings the result, which is why it is read alongside the absolute values rather than in isolation.
A Second Worked Example
A patient on the ward has a BUN of 60 mg/dL and a creatinine of 2.0 mg/dL: 60 ÷ 2.0 = 30:1. The creatinine confirms reduced filtration, but the ratio well above 20:1 adds a pre-renal flavour — perhaps the patient is also volume-depleted on top of underlying kidney disease. Rehydration that brings the BUN down while creatinine improves would support that reading. The ratio turned a single snapshot into a testable hypothesis.
Limitations and Edge Cases
- The ratio is a clue, not a stand-alone test; it never replaces eGFR or creatinine clearance for staging kidney function.
- High-protein meals, corticosteroids, and gastrointestinal bleeding raise BUN independently of kidney function and can inflate the ratio.
- Low muscle mass lowers creatinine, which can artificially raise the ratio in frail or elderly patients.
- Both inputs must be in the same units (mg/dL). Mixing mg/dL and µmol/L produces a meaningless number.
Read more on what the BUN/creatinine ratio means, and estimate creatinine clearance for the full kidney-function picture.