BUN-to-creatinine ratio calculator, with the SI urea ratio

BUN (blood urea nitrogen) and creatinine are on every basic and comprehensive metabolic panel. Both are waste products the kidneys clear, so both rise when filtration falls. But they come from different places: urea from the protein you eat and break down, creatinine from muscle. Dividing one by the other gives the BUN-to-creatinine ratio, which many US reports print as “BUN/Creatinine Ratio”. Enter the two values from the same panel below. If your lab reports urea in mmol/L and creatinine in µmol/L, switch the units.

The formula

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

Labs that use SI units report urea, not urea nitrogen, in mmol/L, and creatinine in µmol/L. Each urea molecule carries two nitrogen atoms, 28.014 g/mol between them, and creatinine’s molar mass is 113.12 g/mol, so:

BUN (mg/dL) = urea (mmol/L) × 2.8

creatinine (mg/dL) = creatinine (µmol/L) ÷ 88.4

UK and European reports often use the urea-to-creatinine ratio instead, with both values in mmol/L. Since creatinine is printed in µmol/L, divide it by 1,000 first:

Urea/creatinine ratio = urea (mmol/L) ÷ creatinine (mmol/L)

The SI ratio is about 4 times the mg/dL ratio. Uchino and colleagues give the traditional cutoff of 20 as 0.081 in international units (urea in mmol/L over creatinine in µmol/L), which is 81 with both in mmol/L.

The second row is a high ratio with a normal creatinine. The third has both values high, but the ratio is ordinary.

What’s a typical ratio

Most references quote a normal ratio of about 10 to 20, as in this case report. Population data fit that. In a study of 42,038 US adults from NHANES, the middle half of ratios ran from 11.4 to 18.6, and a quarter of people were above 18.6. Older people and women tended to have higher ratios. For the SI ratio, Manoeuvrier and colleagues note that it is normally below 100.

So a ratio of 22 on a routine panel, with BUN and creatinine both in range, is not unusual. Roughly a quarter of the adults in that study were above 18.6.

What a high ratio can suggest

The classic reason for a high ratio is low blood flow to the kidneys, as with dehydration or heart failure. Uchino and colleagues describe the theory: the kidneys hold on to more water, and urea is reabsorbed with it, so BUN rises faster than creatinine. Doctors call this prerenal, meaning the problem starts before the kidney. StatPearls’ review of renal function tests puts the textbook numbers on it: when BUN is raised, the ratio is close to 20:1 in prerenal disease and closer to 10:1 when the kidney itself is damaged.

Plenty of other things raise BUN without changing the kidneys. Bleeding in the stomach or upper gut is the best known, since the blood is digested as protein; StatPearls notes it can push the ratio very high, sometimes above 30:1. Manoeuvrier and colleagues also list a high-protein diet, corticosteroids, fever, infection, trauma, an overactive thyroid, and the antibiotic tetracycline. A low creatinine raises the ratio too, as Uchino points out, and creatinine runs low in people with little muscle.

What a low ratio can suggest

A low ratio usually means BUN is low for the creatinine. The liver makes urea, so liver failure can lower it, and so can a diet with little protein. Manoeuvrier and colleagues list both. In rhabdomyolysis, a severe breakdown of muscle, the case report above found the ratio stayed between 3.5 and 11.2 even as both values climbed.

A rough clue, not a diagnosis

The ratio has been used for decades to sort kidney injury into prerenal and intrinsic (damage to the kidney itself). Studies that tested it found it does that poorly.

• Manoeuvrier and colleagues looked at 1,103 emergency patients with a raised creatinine. The ratio averaged about 91 (SI) in both groups, and its ability to tell them apart was no better than a coin toss.
• Uchino and colleagues studied 20,126 hospital patients. Of the 3,641 with kidney injury, about half had a ratio above 20, with no clean split between two groups. Those with a high ratio died more often (29.9% against 18.4%), the opposite of what the prerenal theory predicts, since prerenal injury is the milder kind. They read a high BUN mostly as a sign of how sick someone is.
• A 2026 meta-analysis of 9 studies and 16,412 patients found the ratio caught 74% of prerenal cases but flagged nearly half of the others too, and rated the evidence low to very low certainty.

The ratio is worth reading alongside BUN and creatinine themselves, never instead of them.

When the calculator differs from your report

• Rounding. Labs divide unrounded values, so a difference of 1 is rounding.
• SI or US ratio. A ratio near 80 or 100 is the SI form. The calculator shows both.
• Units. A creatinine of 80 is µmol/L, not mg/dL. The lab unit converter converts either way.

Tracking BUN and creatinine over time

The ratio is a single-day number, and it shifts with what you ate and how much you drank. Creatinine is the steadier line, and it feeds eGFR. How to track your eGFR over time covers what moves creatinine and how much change is real. The eGFR calculator turns a creatinine into an eGFR.

The ratio in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports metabolic panels from lab report PDFs, MyChart downloads, and direct connections to your health system. A ratio your lab prints is filed as BUN/Creatinine Ratio, and in Trends the Kidney tag pulls together BUN, creatinine, and eGFR.

If your lab doesn’t print the ratio, write a derived test: with A as BUN and B as creatinine, both in mg/dL, and a date window of 0 days, the expression is A / B. It’s charted as its own line, apart from any ratio your lab reports.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This calculator is for understanding your lab report, not for diagnosis or treatment. Ask your clinician about an abnormal BUN or creatinine, and what your ratio means for you.