Osmolar Gap Calculator

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 (values 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.