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. Ethanol is added separately using the empirically derived Purssell divisor 3.7. A normal gap does not exclude toxic alcohol exposure. 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. Gap = measured - calculated. Ethanol-adjusted calculated osmolality = calculated + ethanol (mg/dL)/3.7; residual gap = measured - ethanol-adjusted value.
Worked example
Na 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL give 290 mOsm/kg. A measured value of 310 gives a gap of 20. Ethanol 37 mg/dL adds 10, leaving a residual gap of 10.
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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.