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Osmolar Gap Calculator

Find the osmolar gap.

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.

Worked example

A lab reports measured osmolality 310, sodium 140, glucose 90 mg/dL and BUN 14 mg/dL. Sodium term: 2 × 140 = 280. Glucose term: 90 / 18 = 5. BUN term: 14 / 2.8 = 5. The calculated osmolality is 280 + 5 + 5 = 290.00 mOsm/kg. The gap is 310 − 290 = 20.00 mOsm/kg.

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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.

Sources

About this calculator

The osmolar gap is the difference between a patient's measured serum osmolality (from a lab freezing-point osmometer) and the osmolality calculated from routine electrolytes: 2×Na + glucose/18 + BUN/2.8 (sodium in mmol/L, glucose and BUN in mg/dL). A gap under roughly 10 mOsm/kg is normal; a wider gap means something osmotically active is dissolved in the blood that the calculated formula doesn't account for.

Emergency physicians and toxicologists use the osmolar gap to screen for ingestion of toxic alcohols — ethylene glycol, methanol, isopropanol — when a patient presents with an unexplained anion-gap metabolic acidosis, since these substances raise measured osmolality long before they show up on a standard chemistry panel. It's also checked alongside severe hyperglycemia, alcohol intoxication, or renal failure, where an unexpectedly high gap prompts further testing rather than a diagnosis on its own.

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  • Editorial team

    The tool wants US-style units: glucose and BUN in mg/dL, sodium in mmol/L, measured value in mOsm/kg. If your lab reports glucose or urea in mmol/L, convert first or the gap will be wrong. Laboratories also use slightly different calculated formulas, so compare the gap against the reference range printed on your own report. It is not a diagnosis. What units does your laboratory report?

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