Osmolar Gap Calculator

How it is calculated

The tool first estimates serum osmolality from routine labs: calculated = 2 × Na + glucose / 18 + BUN / 2.8. Here Na is sodium in mmol/L, glucose and BUN (blood urea nitrogen) are in mg/dL, and the divisors 18 and 2.8 convert those mass units to osmoles. Then gap = measured − calculated, where measured is the osmometer value in mOsm/kg. Both results are shown with two decimals.

All four values are required and none may be negative. The gap can be negative if the measured value is lower than the estimate. The tool applies this single formula only; other laboratories use variants (for example adding an ethanol term), which will give different gaps. It is a screening figure, not a diagnosis.

Common mistakes

• Entering glucose or BUN in mmol/L; the formula expects mg/dL, so SI values must be converted first or the calculated osmolality will be wrong.
• Typing the calculated osmolality in the measured field, which makes the gap meaningless; the measured value must come from the lab osmometer.
• Using values drawn at different times; a gap is only reliable when the osmolality and the chemistry come from the same sample.
• Reading a gap as a diagnosis on its own, or a small gap as proof that nothing is wrong; it is one clue to be interpreted by a clinician.
• Comparing against a reference range built for a different formula (for example one that includes ethanol) without noting which one the tool uses.