Serum Osmolality Calculator

Serum osmolality describes the concentration of dissolved particles in the liquid portion of blood. It helps clinicians evaluate water balance and investigate disorders involving sodium, glucose, kidney function, toxic alcohol exposure, and altered mental status. A laboratory can measure osmolality directly with an osmometer, while this calculator estimates it from three routinely available chemistry results: sodium, glucose, and blood urea nitrogen, or BUN. The commonly used formula in conventional US units is twice the sodium concentration plus glucose divided by 18 plus BUN divided by 2.8. Sodium contributes most of the calculated value because it and its accompanying anions are the principal extracellular osmoles. The divisors convert glucose and BUN from milligrams per deciliter to millimoles per liter. Results are expressed in milliosmoles per kilogram. Many laboratories use a reference interval near 275 to 295 mOsm/kg, although ranges and calculation conventions vary. Calculated and measured osmolality are not interchangeable. Their difference is called the osmolal gap: measured osmolality minus calculated osmolality. A raised gap can reflect unmeasured osmoles, including ethanol, methanol, ethylene glycol, isopropanol, ketones, mannitol, or other substances. The selected equation matters because some formulas include potassium or use different coefficients. Always compare the result with the equation and reference range used by the reporting laboratory. Timing matters as well: values from different blood draws should not be mixed because treatment or changing physiology may alter each component independently. This calculator expects sodium in mEq/L and glucose and BUN in mg/dL. Using SI glucose or urea values without conversion produces an incorrect answer. Abnormal results should be interpreted with volume status, renal function, acid-base findings, medications, exposures, and repeat laboratory measurements. A normal calculated value does not exclude a clinically important osmolal gap when an unmeasured substance is present. Severe symptoms, marked sodium disturbance, suspected poisoning, or an unexpectedly high osmolal gap requires urgent professional assessment. The output is educational and is not a diagnosis or treatment recommendation.