BUN to Creatinine Ratio Calculator

What the BUN to creatinine ratio measures

Blood urea nitrogen (BUN) and creatinine are two waste products that healthy kidneys clear into urine. They travel together most of the time, yet they do not rise and fall at the same rate, and that gap is the useful part. Divide BUN by creatinine and you get one number that doctors read as a ratio, usually written as something like 14:1.

Urea comes from protein. The liver builds it as it breaks down the protein you eat and the protein your body recycles, and the kidneys filter it out. Creatinine comes from muscle work: it is the breakdown product of creatine phosphate, so the amount in your blood depends mostly on how much muscle you carry and how well your kidneys filter. A muscular 25 year old and a small 80 year old can have different creatinine levels with the same kidney function, which is why the ratio is read next to the two raw numbers, never on its own.

Because the two wastes respond to different things, the ratio hints at where a problem sits. Extra urea from a high protein meal, a slow bleed in the gut or a low-flow state pushes the ratio up. A small muscle mass, or a liver that makes less urea, pulls it down. That is the whole idea behind the tool: one division, then a careful reading of the result.

The formula and the three reading bands

The math is a single division, and both inputs use the same unit:

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

The result has no unit of its own, so it is written with a colon: a BUN of 14 mg/dL with a creatinine of 1.0 mg/dL gives 14, written 14:1. The colon form is just a reading habit. The number behind it is a plain quotient.

A ratio of exactly 20:1 sits at the top edge of the usual band, not outside it. The widget treats the band as closed at both ends: 10 and 20 count as inside. Anything past those two edges gets the high or low label.

One limit belongs up front. The ratio says nothing about how well the kidneys filter on its own. Both numbers can climb together, hold the ratio steady and still show falling filtration. That is why clinicians pair the ratio with eGFR, urine tests and the trend over time.

Where the 10:1 to 20:1 band comes from

The band is a convention, not a law of nature, and it comes from the way the two wastes behave. The MedlinePlus entry on the BUN blood test, reviewed in 2025, gives a usual adult BUN of 6 to 20 mg/dL. The MedlinePlus page on the metabolic panel lists creatinine at 0.6 to 1.3 mg/dL. Put the middle of each range over the other and you land near 13:1, right in the middle of the quoted band.

Clinical writing uses the same edges. A review in American Family Physician describes a BUN to creatinine ratio above 20:1 as a marker that points toward a prerenal cause, meaning the kidney itself is not damaged but blood flow to it is low. The same table puts intrinsic kidney problems at 10 to 20:1. The StatPearls chapter on renal function tests says much the same: close to 20:1 in prerenal disease, closer to 10:1 when the kidney tissue is the problem, and sometimes above 30:1 with bleeding in the upper gut.

The older Clinical Methods chapter on BUN and creatinine makes the underlying point plainly. A patient with a serum creatinine of 5.0 mg/dL would be expected to have a BUN near 50 mg/dL, a ratio of about 10:1. When the BUN comes back at 100 mg/dL instead, the clinician starts looking for factors outside the kidney. That 10:1 expectation is a rule of thumb for moderate to advanced kidney failure, and the chapter says so.

Two practical notes follow. Labs print their own reference intervals, and those intervals move with method, age, sex and muscle mass, so the range on your report is the one that applies to you. And the ratio is not a screening test on its own. It is a reading aid that sits on top of two numbers that already exist.

Worked examples from the calculator

Each row below is what the widget on this page prints for those inputs, and each was checked against the arithmetic by an automated test:

Row two is the classic teaching case. A BUN of 32 mg/dL is above the 6 to 20 mg/dL range, while a creatinine of 1.1 mg/dL is still inside 0.6 to 1.3 mg/dL. Urea rose, creatinine barely moved, so the ratio climbed. Dehydration does this: less water flowing through the kidney means more urea gets pulled back into the blood, while creatinine stays where it is.

Row three runs the other way. A BUN of 9 mg/dL is at the low end and a creatinine of 1.5 mg/dL is above the printed range, so the ratio drops to 6:1. That pattern asks about muscle mass, protein intake and liver function before it asks about the kidney filter.

What pushes the ratio above 20:1

Most high ratios come from one of five situations, and the list is short enough to hold in your head:

The common thread is a kidney that works but sees less blood, or a body that makes more urea than usual. The American Family Physician review of acute renal failure lists a ratio above 20:1 as one of the findings that separates a prerenal problem from damage inside the kidney, alongside urine osmolality above 500 mOsm and a low fractional excretion of sodium.

A high ratio is not a diagnosis. A runner who ate a large steak the night before a blood draw, a person on a water pill for blood pressure and a patient with a slow ulcer bleed can all print the same ratio. The number narrows the question; the history answers it.

What pulls the ratio below 10:1

Low ratios are less discussed and just as informative. The Clinical Methods chapter ties a low BUN to creatinine ratio to four broad causes: not enough protein in the diet, less urea made by a damaged liver, extra urea lost through another route, and more creatinine produced than usual.

Advanced liver disease is the best known. The liver is where urea is built, so a failing liver produces less of it, and the BUN falls even when the kidneys are fine. MedlinePlus makes the same point in plain language: with liver disease, BUN can be low while the kidneys work normally.

Low protein intake and malnutrition work through the same lever from the other end. Less protein in means less urea out, while creatinine holds steady because it comes from muscle turnover, not from dinner. Pregnancy can lower the ratio too, since blood volume rises and dilutes both wastes. A small frame or low muscle mass lowers creatinine and pushes the ratio up rather than down, which is why the two directions are worth keeping straight.

One more cause sits on the creatinine side: rhabdomyolysis, the breakdown of muscle after a crush injury or extreme exertion, releases creatine and lifts creatinine sharply. Urea does not keep pace, so the ratio falls. In that setting the ratio is a side note next to the creatinine rise, which is the finding that matters.

Unit traps and lab reference ranges

Most mistakes with this ratio come from units, not from arithmetic.

The first trap is the urea versus urea nitrogen split. In the United States, labs report only the nitrogen part of the urea molecule, which is about 45 percent of its mass. In much of Europe and Asia, labs report the whole urea molecule in mmol/L. Dividing a whole-urea number by a creatinine in micromoles gives a small decimal near 0.05, and comparing that to 10:1 means nothing.

The second trap is mixing sample types. A serum creatinine and a urine creatinine are different measurements with different reference ranges, even though both are called creatinine. The ratio on this page expects two blood values from the same draw.

The third trap is trusting the band more than the trend. A ratio of 21:1 on one draw, in a person who feels well and drinks normally, means less than a ratio that climbed from 12:1 to 21:1 across two visits. Trends carry information that a single snapshot cannot.

What the calculator returns, and what it cannot tell you

Enter a BUN and a creatinine, both in mg/dL, and the tool returns four things: the ratio written as a colon figure to one decimal place, the band it falls in (below 10:1, 10:1 to 20:1, or above 20:1), a check of each input against the ranges MedlinePlus prints, and a short plain note about the pattern. A creatinine of 0 is caught before the division and reported as undefined rather than as an error.

Everything runs in your browser with plain JavaScript. Nothing is uploaded, no result is stored, and the page works offline once it has loaded. That matters for a health number you may not want to type into a form that travels somewhere.

The tool cannot tell you what your result means for you. A ratio is one line on a metabolic panel, read next to eGFR, electrolytes, urine protein, medicines and symptoms. If a value on your report sits outside the printed range, or if you have swelling, little urine output, confusion or blood in your stool, contact a clinician rather than working it out from a calculator. The numbers here are a reference estimate for understanding a report you already have, not a substitute for care.