BUN Creatinine Ratio Calculator

The BUN/creatinine ratio compares two waste products from a routine blood test: blood urea nitrogen (BUN) and creatinine. Both are cleared by the kidneys, so both rise when the kidneys filter less. The ratio shows whether one has risen more than the other, which doctors read as a clue to the cause, not as a diagnosis.

What the BUN/Creatinine Ratio Measures

BUN comes from urea, which the liver makes when it breaks down protein. Creatinine comes from muscle at a fairly steady rate, which is why women, who often have less muscle, tend to have lower values⁴. BUN is affected much more than creatinine by diet, fluid levels, bleeding in the gut and illness². The ratio mainly tracks those outside effects. It does not measure how well your kidneys filter blood. For that, see your eGFR.

The Formula

BUN/creatinine ratio = BUN (mg/dL) ÷ creatinine (mg/dL)

SI ratio = urea (mmol/L) ÷ creatinine (µmol/L)

BUN (mg/dL) = urea (mmol/L) x 2.8, and creatinine (mg/dL) = creatinine (µmol/L) ÷ 88.4

The conversion factors come from the KDIGO kidney guideline⁷. The 2.8 factor exists because each millimole of urea holds two nitrogen atoms, which weigh 28 mg, and BUN counts only that nitrogen².

How This Calculator Computes Your Result

Pick the unit your lab used for the first value: BUN in mg/dL (US labs), urea in mmol/L (UK, Australia, New Zealand and much of Europe), or urea in mg/dL (used by some labs). Enter creatinine in mg/dL or µmol/L. The tool converts both values to mg/dL, divides, and rounds the ratio to 1 decimal. The band is set from that rounded number.

It also shows the SI ratio to 3 decimals. This is the same result on a different scale. A ratio of 20 in mg/dL equals about 0.081 in SI units⁵.

The tool also checks each value against a normal range: BUN 6 to 20 mg/dL³, and creatinine 0.7 to 1.3 mg/dL for men or 0.5 to 0.95 mg/dL for women⁴. Sex is optional and does not change the ratio. If you leave it blank, the creatinine check uses 0.5 to 1.3 mg/dL. Labs set their own ranges, so go by your report if it differs.

What Your Result Means

A normal BUN/creatinine ratio is usually between 10:1 and 20:1¹. Your result falls into one of three bands. The last column applies only when BUN or creatinine is outside its normal range.

These links come from a lab-test guide, a classic medical text and MedlinePlus¹ ² ³. In moderate to advanced kidney failure, BUN is usually about 10 times creatinine, so the ratio sits near 10².

Why the Ratio Means Little When Both Values Are Normal

The ratio is mainly used to read abnormal results. When both values are normal, small numbers make it swing widely. Using the MedlinePlus ranges (BUN 6 to 20 mg/dL, creatinine 0.5 to 1.3 mg/dL), two normal results can give any ratio from 4.6 to 40.0³ ⁴. A woman with BUN 19 mg/dL and creatinine 0.6 mg/dL has a ratio of 31.7, with both values normal. That is why this tool shows a note, not a band meaning, when both values are within range.

What Can Raise or Lower the Ratio

BUN rises with a high-protein diet, bleeding in the gut, fever or infection, and steroid medicines. It also rises with dehydration or less blood flow to the kidneys, as in heart failure, because the kidneys then take back more urea² ³. BUN falls with liver failure, a low-protein diet, malnutrition and over-hydration, and people with liver disease may have a low BUN even when their kidneys work normally³.

Gut bleeding has the clearest evidence. In a review of studies of people with sudden gut bleeding, a ratio above 30 made a bleed in the upper gut (stomach or upper intestine) more likely than one lower down⁶. Digested blood is a large protein load, which the liver turns into urea¹.

Creatinine matters too. People with little muscle have low creatinine, which can push the ratio up with healthy kidneys⁴.

The Limits of the Ratio

A ratio above 20 is often used to suggest that kidney injury comes from low blood flow rather than damage to the kidney itself. A study of 20,126 hospital patients, 3,641 of whom had kidney injury, found no support for this use⁵. Low blood flow is usually the milder cause, yet patients with a ratio above 20 were more likely to die in hospital (29.9% vs 18.4%)⁵. The ratio was driven mostly by BUN: BUN was 59% higher in the high-ratio group, while creatinine differed by only 6%⁵. The authors also called the cutoff of 20 arbitrary, even though textbooks use it widely⁵.

SI Units and the Urea Mix-Up

The most common mistake is dividing the wrong numbers. Urea in mmol/L and creatinine in µmol/L give a ratio near 0.06, which is normal on the SI scale, not low. Urea in mg/dL is not the same as BUN. It is about 2.1 times higher, because urea weighs more than the nitrogen in it². Dividing urea 60 mg/dL by creatinine 1.2 mg/dL gives 50, but the true BUN/creatinine ratio is 23.3. Choosing the unit on your report avoids both mistakes.