Urea and BUN Unit Converter
Urea and BUN Unit Converter
Convert between BUN in mg/dL, urea in mg/dL and urea in mmol/L — three ways of reporting the same molecule.
Urea and BUN converter
BUN ⇄ ureaBUN 18 mg/dL
The three ways urea is reported
Urea (mg/dL) = BUN (mg/dL) × 2.14
BUN (mg/dL) = urea (mmol/L) ÷ 0.357
- BUN
- blood urea nitrogen — measures only the nitrogen, and each urea molecule contains two nitrogen atoms (28.01 of its 60.06 Da)
- 2.14
- 60.06 ÷ 28.01 — the ratio of whole urea to its nitrogen content
- 0.357
- converts BUN in mg/dL to urea in mmol/L, from the nitrogen molecular weight of 28.01
- convention
- the United States reports BUN in mg/dL; most other countries report urea in mmol/L
Worked example
BUN 18 mg/dL
18 × 0.357 = 6.43 mmol/L urea
18 × 2.14 = 38.5 mg/dL urea
Equivalent values
| BUN mg/dL | Urea mg/dL | Urea mmol/L |
|---|---|---|
| 7 | 15.0 | 2.5 |
| 18 | 38.5 | 6.4 |
| 20 | 42.8 | 7.1 |
| 40 | 85.6 | 14.3 |
| 80 | 171.2 | 28.6 |
Why there are three numbers for one molecule
Urea is the end product of protein catabolism, made in the liver from ammonia and excreted by the kidneys. The confusion in reporting it comes from history: American laboratories measure and report the nitrogen content — blood urea nitrogen — while most of the rest of the world reports the whole urea molecule in mmol/L. Each urea molecule contains two nitrogen atoms, 28.01 of its 60.06 Da, so urea is 2.14 times BUN by mass.
Mistaking one for the other is a genuine and recurring clinical error, particularly when a patient moves between health systems or when a paper written in one convention is applied in another. A BUN of 18 mg/dL and a urea of 18 mg/dL differ by more than twofold.
Urea is a poorer filtration marker than creatinine because roughly 40 to 50% of filtered urea is reabsorbed, and that fraction rises when tubular flow slows in volume depletion. This is exactly what makes the BUN-to-creatinine ratio useful: a disproportionate rise in urea points to a pre-renal state. Urea also rises with gastrointestinal bleeding, high protein intake, corticosteroids and catabolic illness, and falls in liver disease, malnutrition and pregnancy — all independently of kidney function.
Frequently asked questions
What is the difference between urea and BUN?
BUN measures only the nitrogen in urea. Urea in mg/dL equals BUN in mg/dL multiplied by 2.14, because nitrogen accounts for 28.01 of urea’s 60.06 Da.
How do I convert BUN to mmol/L?
Multiply BUN in mg/dL by 0.357. A BUN of 18 mg/dL is 6.4 mmol/L of urea. To go back, divide the mmol/L value by 0.357.
Is urea a good measure of kidney function?
Less good than creatinine. Around half of filtered urea is reabsorbed, and that fraction rises in volume depletion, so urea reflects hydration, protein intake and catabolic state as well as filtration.
Why is my urea high but creatinine normal?
A disproportionate rise in urea suggests a pre-renal state, gastrointestinal bleeding, high protein intake, corticosteroids or a catabolic illness. Calculating the BUN-to-creatinine ratio makes the pattern explicit.
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References
- KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics, urea and urea nitrogen.