How this calculator works
The conventional-unit equation is applied after converting SI values to mg/dL. The SI urea field is urea concentration, while the conventional field is blood urea nitrogen (BUN), not total urea mass. The divisors 18 and 2.8 are the published rounded clinical coefficients, so equivalent SI inputs can differ slightly from the simple SI shortcut. This is an estimate of osmolality rather than a freezing-point measurement. The result is an estimate for information only and is not medical advice.
Formula
Calculated osmolality = 2 Na + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Rearrange this expression for Na, glucose or BUN. Na is mmol/L; osmolality is mOsm/kg.
Worked example
Na 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL give 290 mOsm/kg. With Na 130, glucose 180 and BUN 28, the result is 280 mOsm/kg.
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Frequently asked questions
Is urea the same as BUN?
No. BUN reports the mass of nitrogen within urea. One mmol/L of urea corresponds to approximately 2.80134 mg/dL BUN.
Why are there several osmolality formulas?
Different empirical equations have different calibration and reference intervals. Use the formula adopted by the reporting laboratory.
Can this identify an unmeasured substance?
No. The calculation is one clue, and interpretation requires laboratory testing and clinical assessment.