What Is Normal BUN And Creatinine Ratio: Full Chart

A normal BUN/creatinine ratio is roughly 10 to 20 when both values are in the American convention of milligrams per decilitre. A normal blood urea nitrogen for an adult is about 6 to 20 mg/dL, and a normal creatinine is about 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for women. If your report uses SI units instead, the equivalent numbers are urea 2.5 to 7.1 mmol/L, creatinine 62 to 115 µmol/L in men and 53 to 97 µmol/L in women, and a urea:creatinine ratio of roughly 40 to 100. Those are the headline figures. The rest of this page is about what they actually mean and where they do not apply.

Two things trip people up immediately. The first is that the ratio has a wider normal band than either number that feeds it, which feels wrong until you see why. The second is that two people can post exactly the same ratio while one has completely healthy kidneys and the other is in trouble, because the ratio deliberately throws away the information about how big the numbers were. If you want the concept and the calculation itself rather than the reference values, what the BUN/creatinine ratio is covers that ground properly. This page stays on the numbers.

Three numbers, three separate normal ranges

The question in the title asks about the ratio, but almost everyone who searches it is holding a report with three numbers on it. BUN. Creatinine. And a ratio the analyser worked out by dividing one by the other. Each has its own reference range, and each range was built differently.

BUN measures the nitrogen carried in urea, the waste product your liver makes when it breaks down protein. It rises and falls with a great many things that have nothing to do with your kidneys: what you ate, how much you drank, whether you are bleeding somewhere, whether you are on steroids. Creatinine comes from muscle, at a rate that stays remarkably steady from one day to the next in the same person. That difference in behaviour is the entire reason the ratio exists.

So the three ranges are not three views of the same thing. The BUN range is wide because BUN is volatile. The creatinine range is narrow and splits by sex because muscle mass splits by sex. And the ratio range is wider still, for a reason that takes a bit of arithmetic to see, which is covered further down.

Hold that contrast in mind while you read the tables. It explains why the BUN column moves so much more between age bands than the creatinine column does, and it explains why the ratio can shift substantially while your kidney function has not changed at all.

Normal BUN values, by sex and by age

The most widely printed adult range for blood urea nitrogen is 6 to 20 mg/dL. Plenty of laboratories print 7 to 20, or 8 to 23, or 6 to 24. These are approximate figures and they differ between analysers and reference populations, so treat the table below as typical rather than definitive, and always read your own report’s printed range first.

Notice how much ground that table covers. The lower end for a newborn is a quarter of the upper end for a man over 60. Almost none of that spread is about kidney health. It is about how much protein is going in and how much muscle and body water is there to distribute it through.

The other thing worth flagging is the direction of the sex difference. Men sit higher on both BUN and creatinine. Because the ratio divides one by the other, those two shifts partly cancel, which is why the normal ratio band is usually printed without any sex split at all. That cancellation is not perfect, but it is close enough that no major laboratory bothers separating it.

Normal urea in mmol/L, and why it is not the same number as BUN

If your report comes from the United Kingdom, Ireland, Australia, most of Europe, Canada or India, it probably does not say BUN at all. It says urea, and it reports millimoles per litre. A typical adult range is 2.5 to 7.8 mmol/L, though 2.5 to 6.7 and 3.0 to 8.0 are both common prints.

These are not two ways of writing the same measurement. BUN measures only the nitrogen atoms in the urea molecule. Urea measures the whole molecule. A urea molecule contains two nitrogen atoms out of a molecular weight of 60, so the nitrogen fraction is 28/60, or about 0.467. Combine that with the unit change from mg/dL to mmol/L and you get the conversion factor everyone quotes: multiply BUN in mg/dL by 0.357 to get urea in mmol/L, or multiply urea in mmol/L by 2.8 to go the other way.

Get this wrong in the direction people usually get it wrong and a perfectly ordinary urea of 5.5 mmol/L becomes a “BUN of 5.5”, which reads as low. It is not low. It is 15.4 mg/dL, which sits comfortably mid-range. This single confusion probably generates more unnecessary worry than any other unit problem in routine blood testing, and it becomes a much bigger problem when you go on to calculate a ratio, as the next sections show.

Normal creatinine values, by sex and by age

Creatinine behaves very differently. The adult range is narrow, it splits by sex, and it barely moves from week to week in a stable person. Standard adult figures are 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for women, which convert to roughly 62 to 115 and 53 to 97 µmol/L respectively.

The conversion between the two creatinine units is a straight multiplication by the molecular weight, 113.12, adjusted for the decilitre-to-litre step. In practice everyone uses 88.4.

One point that matters more than it sounds: a creatinine of 1.0 mg/dL is unremarkable in a 95 kg man with visible muscle and genuinely borderline in a 48 kg woman of 80. Same number, different meaning, because the production side of the equation is not the same. That is the single biggest limitation of reading creatinine against a printed range, and it carries through into the ratio. There is a fuller treatment in what a normal creatinine level is, including how the ranges are set for each age band, and creatinine levels for women deals with the sex difference specifically.

The normal BUN/creatinine ratio in mg/dL

With both components in mg/dL, the calculation is a plain division and the widely accepted normal band is 10:1 to 20:1. Some laboratories print 6 to 22, some print 10 to 20, a few print 12 to 20. The most commonly used figure in clinical practice, and the one most likely to appear on your report, is 10 to 20.

Those bands are pointers, not diagnoses. A ratio of 21 does not mean something is wrong, and a ratio of 19 does not mean everything is fine. The bands either side of normal are covered separately in what a dangerously high ratio means and what a low ratio means, which is why this page does not chase them.

The normal urea:creatinine ratio in SI units

Here is where most of the confusion in this topic lives. If you take a urea in mmol/L and divide it by a creatinine in µmol/L, you get a number like 0.06, which matches nothing you have read anywhere. So the SI convention converts creatinine to millimoles first, and the resulting ratio runs on a completely different scale from the American one.

The normal SI urea:creatinine ratio is usually quoted as roughly 40 to 100. Some sources use 40 to 80, others 50 to 100, and a threshold above 100 is often treated as suggestive of a pre-renal picture. That spread of published cut-offs is real and worth knowing about, because it means an SI ratio of 95 might be flagged by one laboratory and not another.

The relationship between the two conventions is fixed, and you can move between them with one multiplication. Multiply the mg/dL ratio by about 4.04 to get the SI ratio, or divide the SI ratio by 4.04 to get the mg/dL one. That single number falls straight out of the two conversion factors, and the next section shows the arithmetic in full.

Look at the row for 20. The American upper limit of normal converts to 81, yet the SI band is often quoted up to 100. The two conventions do not line up exactly, and neither is wrong. They were derived from different reference populations using different statistical cut-offs, and the SI band tends to be drawn a little more generously at the top. If you calculate your own ratio in one system and compare it to a range published in the other, you can talk yourself into a problem that does not exist.

The conversion arithmetic, written out step by step

Most pages tell you to multiply by 4.04 and leave it there. It is worth seeing where that comes from, because once you have seen it you will never again mix up a urea with a BUN.

How the three numbers interact, and why identical ratios can mean opposite things

This is the part that reference tables cannot show you. A ratio is a relationship, and it discards the scale. Two people can hand you the same ratio while sitting in completely different clinical situations, because the ratio only knows how the two numbers relate to each other, not how big they were.

Both ratios are 14. Both sit dead centre of the normal band. Nobody would look twice at either ratio in isolation, and yet one of these people has entirely healthy kidneys and the other has advanced chronic kidney disease. The ratio was never designed to detect that. It was designed to answer a narrower question: given that something is off, is the pattern more consistent with reduced blood flow to the kidney or with damage inside it?

The practical rule follows directly. Read the ratio last. Look at the creatinine first, because that is your filtration signal. Look at the BUN second. Only then does the ratio tell you anything, and what it tells you is about the relationship, not the severity. If your creatinine is raised, the useful reading is in what a high creatinine means and what causes high creatinine levels, not in the ratio line.

Why the normal band for a derived ratio is wider than for either component

A quick look at the numbers makes this obvious, and it explains something that puzzles a lot of people: how can a ratio be abnormal when both of the numbers that produced it are normal?

Take the adult normal ranges at their extremes. BUN can legitimately be anywhere from 6 to 20 mg/dL. Creatinine can legitimately be anywhere from 0.6 to 1.3. Now divide the extremes against each other.

So a person whose BUN and creatinine are both inside their reference ranges can produce a ratio anywhere from about 5 to about 33, which stretches well past both ends of the ratio’s own normal band. This is not a flaw in the ranges. It is what happens whenever you divide one variable quantity by another: the uncertainties compound rather than cancel.

Two things follow. First, an out-of-range ratio with two in-range components is common and usually means very little. A slim woman with a BUN of 19 and a creatinine of 0.65 has a ratio of 29, and both her numbers are perfectly normal. Second, the ratio’s normal band is deliberately drawn wide, because a narrow band would flag enormous numbers of healthy people. Even at 10 to 20, the ratio produces more false alarms than either of its components does alone.

How reference ranges are actually built

A reference range is not a statement about health. It is a statement about a group of people who were tested once, and understanding how it was made explains most of the odd things about it.

The standard method takes a reference population, usually a few hundred people screened to exclude known disease, measures them all, and then draws the range so that the middle 95% of results fall inside it. That is the whole recipe. The consequence built into it is that 5% of perfectly healthy people fall outside by definition, half of them above and half below. Nothing has gone wrong when that happens. The range was constructed to make it happen.

For a derived quantity like the BUN/creatinine ratio, the process is messier still. Many laboratories do not derive a ratio range from their own population at all. They print the conventional 10 to 20 because it is what the literature uses and what clinicians expect. That is a reasonable practical choice, but it does mean the ratio range on your report may not have been validated against the same people whose blood set the BUN and creatinine ranges printed two lines above it.

Why your laboratory prints different numbers from the ones you looked up

You searched the range, found 10 to 20, and your report says 6 to 22. Neither is wrong. Several separate things drive the variation, and they stack.

The practical rule is short. Compare your result to the range printed on the same page as the result, not to a range you found elsewhere; the MedlinePlus page on the creatinine test makes the same point about variation between laboratories. And if you are comparing this year’s test with one from a different laboratory, check that both were reported in the same units and, ideally, on the same method. Otherwise you may be measuring a change in analyser rather than a change in you.

The same principle applies to filtration estimates, which are calculated from creatinine and inherit every one of these sources of variation. Creatinine clearance compared with GFR goes through how the two differ and why the estimate can move when your kidneys have not.

Pregnancy: all three numbers move, and they move downwards

Pregnancy changes kidney physiology more than almost any other normal state, and the standard adult ranges stop applying from the first trimester onwards. Plasma volume expands substantially. Glomerular filtration rate climbs by something in the region of 40 to 50% by mid-pregnancy. Both effects push urea and creatinine down.

The clinically important consequence is the opposite of what you might expect. Because the normal creatinine in late pregnancy is so much lower, a value that would be reassuring in anyone else can be a warning sign in pregnancy. A creatinine of 1.0 mg/dL is entirely normal for a non-pregnant woman and distinctly abnormal at 32 weeks, where it may point to pre-eclampsia or another problem needing prompt assessment. Reading a pregnant woman’s result against the standard adult range is one of the more consequential mistakes in this whole area. Creatinine levels in pregnancy covers the trimester-by-trimester picture in detail.

Older adults: the range stays the same while the meaning changes

Most laboratories print the same creatinine range for a 30-year-old and an 85-year-old. That is defensible, and it is also the reason a normal creatinine in an older person deserves more scepticism than a normal creatinine in a young one.

Filtration declines with age. From roughly the fourth decade onwards, GFR falls at a rate often quoted around 0.8 to 1 mL/min per year, though the spread between individuals is enormous and plenty of healthy older people barely decline at all. At the same time, skeletal muscle mass falls. Less muscle means less creatinine produced. The two changes push the blood level in opposite directions and they broadly cancel, so the number on the report stays put while the kidney behind it is doing considerably less work.

This is exactly why estimated filtration matters more than raw creatinine as people get older. Running the number through the creatinine clearance calculator, which factors in age, sex and weight, will often show a clearance well below what the “normal” creatinine implied. What creatinine clearance is explains what the output actually represents, and the normal creatinine clearance range gives the expected values by age.

Children: why adult ranges must never be used

Paediatric creatinine tracks body size, not age, and applying an adult range to a child produces nonsense in both directions. A creatinine of 0.4 mg/dL is normal for a four-year-old and would be flagged low in an adult. A creatinine of 0.9 mg/dL would be unremarkable in an adult woman and, in a five-year-old, would represent a substantial reduction in filtration that needs investigating.

Look at the ratio column for infants. Because the creatinine denominator is so small, the calculated ratio in young children is routinely above the adult normal band without anything being wrong. Paediatric practice generally does not use the BUN/creatinine ratio the way adult practice does, precisely for this reason. If a child’s report shows a ratio flagged high, that flag was almost certainly generated against an adult reference band and needs a paediatric interpretation before anyone reads anything into it.

Protein intake, and how much it moves the numbers

Urea is the end product of protein metabolism, so how much protein you eat changes your BUN directly. Creatinine barely responds. That asymmetry means diet alone can move the ratio across the normal band and out the other side, in either direction, with completely healthy kidneys.

The practical implication is that a single ratio outside the band, in someone who feels well and whose creatinine is normal, is very often a diet-and-hydration story rather than a kidney story. Repeating the test after a few normal days of eating and drinking settles most of these. If yours is running high and you want the ordinary explanations first, how to lower a BUN/creatinine ratio works through them, and whether dehydration raises creatinine covers the most common single cause.

What a normal ratio does, and does not, tell you

Worth being blunt here, because this is where the number gets over-read in both directions.

The mistake that follows from misunderstanding this is common enough to name. Someone sees “BUN/creatinine ratio: 15 (normal)” on a report where the creatinine is 2.4 mg/dL, and takes reassurance from the word normal. The ratio is normal. The kidney function is not. Reading the ratio without reading its components is the single most frequent error people make with this line of a blood panel. NIDDK’s guidance on kidney tests puts the emphasis where it belongs, on the filtration estimate and urine albumin rather than on any single derived figure.

The reverse error is just as common and less harmful. Someone sees a ratio of 23 flagged high, with a BUN of 21 and a creatinine of 0.9, and worries. Both components are within range. The person had a busy week, drank little, and ate well. Repeat it in a fortnight and it will very likely be 17. When to actually worry about creatinine levels sets out which patterns justify concern and which do not.

Borderline values: 9, 21, 22 and the space around the edges

Reference ranges have hard edges and biology does not. A ratio of 20 is normal and a ratio of 21 is flagged, and there is no meaningful difference between the two people who produced them.

Analytical variation alone accounts for a few percent on both measurements, and biological variation within the same person across a week accounts for more. Put those together and a ratio calculated from a single blood draw carries an uncertainty of several units. Treating 20 as a cliff edge misrepresents the precision of the underlying test.

What does deserve attention is a consistent direction of travel. Three tests over eighteen months showing ratios of 13, 17 and 22, with creatinine also creeping up, is a different proposition from one isolated 22. Direction beats position. That principle applies across the whole panel, not just here.

Three reports, read end to end

Reference tables become useful once you have watched someone use them. Here are three, each read in the order a clinician would read it: creatinine first, BUN second, ratio last.

Three reports, three flagged or unflagged ratios, and in each case the ratio was the last thing that mattered. That is the correct order of reading, and it is worth practising on your own results. If you have your creatinine to hand, running it through the CrCl calculator alongside your age, sex and weight gives you the filtration estimate that the raw number alone cannot.

Normal BUN and creatinine ratio: frequently asked questions