BUN vs Urea: Why the Factor Is Not 1:1
This is the key point most converters skip. BUN (blood urea nitrogen), the US convention, measures only the two nitrogen atoms in the urea molecule. Urea, used in most other countries, measures the whole molecule (molecular weight 60 vs 28 for the nitrogen). So they are different quantities of the same thing, and you must convert rather than assume they are equal.
Urea mmol/L = BUN mg/dL × 0.357 BUN mg/dL = urea mmol/L ÷ 0.357 (≈ × 2.8) Urea mg/dL = BUN mg/dL × 2.14 BUN mg/dL to Urea mmol/L Chart
| BUN (mg/dL) | Urea (mmol/L) | Typical band |
|---|---|---|
| 7 mg/dL | 2.5 mmol/L | Low-normal |
| 10 mg/dL | 3.6 mmol/L | Normal |
| 15 mg/dL | 5.4 mmol/L | Normal |
| 20 mg/dL | 7.1 mmol/L | Upper normal |
| 30 mg/dL | 10.7 mmol/L | Elevated |
| 50 mg/dL | 17.9 mmol/L | High |
Urea mmol/L to BUN mg/dL Reverse Chart
| Urea (mmol/L) | BUN (mg/dL) |
|---|---|
| 2.5 mmol/L | 7.0 mg/dL |
| 5.0 mmol/L | 14.0 mg/dL |
| 7.1 mmol/L | 19.9 mg/dL |
| 10.0 mmol/L | 28.0 mg/dL |
Reference Ranges
A typical adult BUN is about 7–20 mg/dL, equivalent to a urea of roughly 2.5–7.1 mmol/L. Ranges vary with age, hydration, protein intake and the laboratory. BUN and urea rise with reduced kidney function, dehydration, high protein intake, or gastrointestinal bleeding, and fall with low protein intake or liver disease.
Urea, Creatinine and Kidney Function
Urea and BUN are read alongside creatinine and estimated GFR, not in isolation. The BUN-to-creatinine ratio helps distinguish dehydration from intrinsic kidney problems. Convert creatinine with the creatinine mg/dL to µmol/L calculator, estimate filtration with the eGFR calculator, or check clearance with the creatinine clearance calculator.
Worked Example
Suppose your report shows BUN 28 mg/dL and you want urea in mmol/L:
28 × 0.357 = 10.0 mmol/L urea 10.0 mmol/L is above the usual upper limit, prompting review of hydration and kidney function. Reversing, a urea of 5.0 mmol/L equals a BUN of about 14 mg/dL.
Frequently Asked Questions
How do you convert BUN to urea mmol/L?
Multiply BUN mg/dL by 0.357. For example, 15 mg/dL is about 5.4 mmol/L.
Are BUN and urea the same?
No. BUN measures only the nitrogen; urea measures the whole molecule, so they need converting.
What is a normal BUN?
Roughly 7–20 mg/dL (urea 2.5–7.1 mmol/L), varying by lab.
What raises BUN or urea?
Reduced kidney function, dehydration, high protein intake, or GI bleeding.
References
- Medscape — Blood Urea Nitrogen (BUN): Reference Range and Interpretation.
- BUN to urea conversion factor 0.357 ([10 dL/L] ÷ [28 mg N/mmol urea]); urea mg/dL = BUN × 2.14 (MW 60 ÷ 28).
Conclusion
Converting BUN to urea is a ×0.357 to mmol/L — but only because BUN and urea measure different parts of the same molecule. Get the direction and the units right, and read the result alongside creatinine and eGFR.