Serum Osmolality Calculator

Calculated Serum Osmolarity

Serum osmolarity reflects the total concentration of osmotically active particles in plasma and is one of the primary regulators of antidiuretic hormone (ADH) secretion and thirst. The classic bedside estimate is Osm = 2·Na + glucose/18 + urea/2.8 in mOsm/L, with glucose and urea (BUN) expressed in mg/dL. Sodium is doubled to account for its accompanying anion (mostly chloride and bicarbonate).

The normal range is roughly 275–295 mOsm/L. The difference between measured (freezing-point depression) and calculated osmolarity is the osmolar gap; values > 10 mOsm/L suggest the presence of unmeasured osmoles such as methanol, ethanol, ethylene glycol, isopropanol, mannitol, glycerol, or accumulated lactate in shock.

Applications

Differential diagnosis of metabolic coma, suspected toxic alcohol poisoning, hyperosmolar hyperglycemic state, evaluation of hyponatremia (true vs pseudohyponatremia), monitoring of mannitol therapy in raised intracranial pressure, and screening prior to fomepizole or hemodialysis decisions in toxicology.

FAQ

Osmolarity or osmolality? Osmolality is per kg of water (lab measurement); osmolarity is per L of solution (calculated). In healthy plasma the two are numerically very close; this calculator returns osmolarity in mOsm/L.

Why divide glucose by 18 and urea by 2.8? Those are the molecular weights divided by 10 (glucose 180 g/mol, urea/BUN 28 g/mol of nitrogen), converting mg/dL into mmol/L which equals mOsm/L for non-dissociating solutes.

Is this a clinical diagnosis? No. This educational tool does not replace direct osmolality measurement or specialist evaluation. Toxic alcohol exposure is a medical emergency — always contact a poison control center.