BUN / urea mg/dL BUN to mmol/L urea Converter

BUN and urea: the short answer

Blood urea nitrogen (BUN) and urea measure the same substance, but in different ways and often in different units, so converting between them takes two steps rather than one. The quickest rules are:

• BUN (mg/dL) → urea (mmol/L): multiply by 0.357. A BUN of 14 mg/dL is a urea of 5.0 mmol/L.
• Urea (mmol/L) → BUN (mg/dL): multiply by 2.8. A urea of 7.0 mmol/L is a BUN of 19.6 mg/dL.
• BUN (mg/dL) → urea (mg/dL): multiply by 2.14. A BUN of 10 mg/dL corresponds to 21.4 mg/dL of urea.

The converter above handles the conversion between BUN in mg/dL and urea in mmol/L, the most common pairing when comparing US and international reports. This guide explains why there are two names, why the factors look odd, and how to read the result.

What BUN and urea measure

When the body breaks down protein, the nitrogen from amino acids is released as ammonia, which is toxic. The liver converts ammonia into urea, a small, water-soluble molecule, through the urea cycle. Urea is carried in the blood to the kidneys, filtered by the glomeruli, partly reabsorbed by the tubules, and excreted in urine. Urea makes up most of the nitrogen in urine.

The blood level of urea depends on three things: how much is produced (protein intake and breakdown, and liver function), how well the kidneys filter it, and how much is reabsorbed (which increases when the body is dehydrated). This makes urea a useful but less specific kidney marker than creatinine.

Why two names

In the United States, laboratories historically measured only the nitrogen content of urea and reported it as blood urea nitrogen (BUN) in mg/dL. Most other countries measure the whole urea molecule and report it in mmol/L. Some countries, including parts of Europe and Latin America, report whole urea in mg/dL, which adds a third format to the mix. They all describe the same substance; only the way of counting differs.

Why the factors are 0.357, 2.8 and 2.14

Each urea molecule (CO(NH2)2, molar mass 60.06 g/mol) contains two nitrogen atoms, which together weigh 28.02 g/mol.

• BUN mg/dL to urea mmol/L: 1 mg/dL of nitrogen is 10 mg per liter. Dividing by 28.02 mg/mmol gives 0.357 mmol of urea per liter, since each mmol of urea carries 28.02 mg of nitrogen. So the factor is 0.357.
• Urea mmol/L to BUN mg/dL: the reverse, 1 ÷ 0.357 = 2.8.
• BUN mg/dL to urea mg/dL: urea weighs 60.06 ÷ 28.02 = 2.14 times as much as its nitrogen.
• Urea mg/dL to urea mmol/L: divide by 6.006 (since 1 mmol/L of urea = 6.006 mg/dL).

A molar concentration of urea is the same as a molar concentration of urea nitrogen measured as molecules, which is why some reports describe urea nitrogen in mmol/L with the same number as urea.

How to convert step by step

1. Identify what your report shows: BUN (urea nitrogen) or urea, and in which unit.
2. BUN in mg/dL → urea in mmol/L: multiply by 0.357.
3. Urea in mmol/L → BUN in mg/dL: multiply by 2.8.
4. Urea in mg/dL → BUN in mg/dL: divide by 2.14.
5. Urea in mg/dL → urea in mmol/L: divide by 6.0.

Worked examples

• BUN 7 mg/dL × 0.357 = urea 2.5 mmol/L – the lower end of a typical adult range.
• BUN 20 mg/dL × 0.357 = urea 7.1 mmol/L – the upper end of a typical BUN range.
• BUN 40 mg/dL × 0.357 = urea 14.3 mmol/L.
• Urea 5.5 mmol/L × 2.8 = BUN 15.4 mg/dL.
• Urea 10 mmol/L × 2.8 = BUN 28 mg/dL.
• Urea 40 mg/dL ÷ 2.14 = BUN 18.7 mg/dL; and 40 ÷ 6.0 = 6.7 mmol/L.

BUN to urea conversion chart

Normal BUN and urea ranges

A typical adult BUN range is about 7–20 mg/dL, equivalent to a urea of about 2.5–7.1 mmol/L. Many laboratories outside the US quote urea ranges of roughly 2.5–7.8 mmol/L (BUN about 7–22 mg/dL). Whole urea in mg/dL ranges from about 15–45 mg/dL.

Differences by age and sex

• Older adults often have slightly higher values, partly because kidney function declines with age; upper limits of 23–25 mg/dL BUN (8–9 mmol/L urea) are used for people over 60 by some laboratories.
• Men tend to have slightly higher values than women.
• Children typically have lower values, particularly infants, because growth uses protein for building tissue.
• Pregnancy lowers urea because of increased kidney filtration and blood volume.

What a high BUN or urea means

A raised level is called azotemia (or uraemia when it causes symptoms). Causes are usually grouped into three categories:

• Prerenal – reduced blood flow to the kidneys: dehydration, vomiting or diarrhoea, heart failure, blood loss or shock. The kidneys reabsorb more urea when blood flow falls, so urea often rises more than creatinine.
• Renal – damage to the kidneys themselves: chronic kidney disease, acute kidney injury from medicines or infection, glomerulonephritis.
• Postrenal – blockage of urine flow: enlarged prostate, kidney stones, tumours.

Urea can also rise without any kidney problem when production increases: a high-protein diet, bleeding into the upper gut (where blood is digested as protein), corticosteroids, tetracyclines, severe burns or a catabolic state such as serious infection.

The BUN/creatinine ratio

Comparing urea with creatinine helps distinguish these causes. A BUN-to-creatinine ratio above about 20:1 (both in mg/dL) suggests a prerenal cause or gut bleeding; a ratio of 10–20:1 is typical of normal kidneys or intrinsic kidney disease. In SI units the equivalent is a urea-to-creatinine ratio above about 80–100:1 (urea in mmol/L, creatinine in mmol/L). Because the ratio depends heavily on units, our separate BUN/creatinine ratio calculator handles the conversion for you.

What a low BUN or urea means

Low values are less common and usually not serious. They may be seen with:

• low protein intake or malnutrition
• severe liver disease, which reduces urea production
• pregnancy
• over-hydration or SIADH
• rare inherited urea-cycle disorders (usually with very high ammonia)

What affects BUN and urea results

• Hydration is the biggest short-term influence; dehydration can double urea with healthy kidneys.
• Recent protein intake – a large protein meal raises urea for several hours.
• Gastrointestinal bleeding raises urea, sometimes markedly.
• Medicines – corticosteroids, tetracyclines and some diuretics raise urea; ACE inhibitors and ARBs can raise it via reduced filtration pressure.
• Muscle breakdown, fever, infection and burns increase production.
• Liver disease lowers production and can hide kidney dysfunction.

Because so many factors affect it, urea is rarely used alone to judge kidney function; eGFR based on creatinine (and sometimes cystatin C) is the standard.

Urea in dialysis

For people on haemodialysis, urea is used to measure how well each session clears waste. The urea reduction ratio (URR) compares urea before and after a session; a URR of at least 65% (and often a Kt/V of at least 1.2) is a common adequacy target. These calculations use the same units before and after, so conversion is not needed, but it helps to know that pre-dialysis urea values of 20–30 mmol/L (BUN 56–84 mg/dL) are common in this setting.

Converting between BUN and urea in hospital practice

Clinicians often need to convert when using guidelines and risk scores developed in a different country. A few common examples:

• The CURB-65 pneumonia severity score, developed in the UK, includes a urea above 7 mmol/L. In US units, that is a BUN above about 19.6 mg/dL – often rounded to 20 mg/dL.
• The Glasgow-Blatchford score for upper gastrointestinal bleeding uses urea bands starting at 6.5 mmol/L (BUN about 18.2 mg/dL), with higher points at 8, 10 and 25 mmol/L (BUN about 22.4, 28 and 70 mg/dL).
• Many US calculators, including the BUN/creatinine ratio, expect BUN in mg/dL; entering urea in mmol/L by mistake gives a ratio almost three times too low.

When you see a guideline threshold without a unit, check its country of origin before applying it.

Urea in the context of the metabolic panel

BUN or urea is usually reported alongside sodium, potassium, chloride, bicarbonate, glucose and creatinine. Several calculated values depend on it:

• Serum osmolality is commonly estimated as 2 × sodium + glucose + urea when all three are in mmol/L. In US units, the formula becomes 2 × sodium + glucose/18 + BUN/2.8. The division by 2.8 is exactly the BUN-to-urea conversion.
• The osmolal gap compares measured with calculated osmolality and depends on the same conversion.
• The BUN-to-creatinine ratio compares the two waste products and helps distinguish prerenal from renal causes of a high level.

Getting the BUN-to-urea conversion right is therefore important for these calculations as well as for comparing single results.

Tracking urea over time

Because urea fluctuates with diet and hydration, a single mildly raised value in a well person is often normal on repeat. A steady rise across several tests, especially together with a rising creatinine, is more meaningful. If your results come from a US laboratory and from a laboratory abroad, convert all of them to one format first. For example, BUN values of 16, 19 and 23 mg/dL followed by urea values of 9.2 and 10.1 mmol/L are, in BUN terms, 16, 19, 23, 26 and 28 mg/dL – a steady upward trend that is easy to miss when the units change halfway through.

Urea and creatinine: how they differ as kidney markers

Both urea and creatinine are filtered by the kidneys, but they behave differently, and knowing the differences helps explain results that seem to disagree:

• Production. Creatinine is made at a steady rate from muscle; urea production varies from day to day with protein intake, bleeding into the gut and illness.
• Reabsorption. Creatinine is barely reabsorbed once filtered. Up to about half of filtered urea is reabsorbed in the tubules, and more is reabsorbed when urine flow is low, as in dehydration.
• Liver dependence. Urea is made in the liver, so severe liver disease lowers urea even when kidney function is poor. Creatinine is unaffected by liver function, although people with cirrhosis often have low muscle mass.
• Use in eGFR. Modern eGFR equations use creatinine and cystatin C, not urea, because urea is too variable.

In practice, urea is most useful for judging hydration and as part of the BUN/creatinine ratio, and for monitoring people with advanced kidney disease and on dialysis.

Typical BUN and urea values by situation

The table below shows representative values to help put a result in context. These are illustrations, not diagnostic cut-offs; interpretation always depends on creatinine and the clinical picture.

Symptoms of a very high urea

Mild rises in urea cause no symptoms. When kidney function falls far enough for urea and other waste products to accumulate – usually with a BUN above about 60–100 mg/dL (21–36 mmol/L) – symptoms of uraemia may appear: tiredness, loss of appetite, nausea and vomiting, a metallic taste, itching, muscle cramps, restless legs, difficulty concentrating, and in severe cases confusion, seizures, bleeding tendency and inflammation of the lining around the heart (pericarditis). These symptoms are due to the overall build-up of toxins rather than urea alone, but urea is a convenient marker of it. Uraemic symptoms, together with eGFR and other factors, help doctors decide when to start dialysis.

When to see a doctor

See a doctor if your BUN is above about 25 mg/dL (urea above 9 mmol/L), especially together with a raised creatinine, or if it is rising compared with previous tests. Seek urgent care for a high urea with reduced urine output, confusion, persistent vomiting, black tarry stools (possible gut bleeding), or signs of severe dehydration such as dizziness on standing and a dry mouth. A very high urea – above about 100 mg/dL BUN (36 mmol/L) – can cause nausea, itching, confusion and inflammation around the heart and needs prompt treatment.