Plasma Osmolality Calculator
Calculate plasma osmolality from sodium, glucose, and BUN levels using the standard clinical formula.
Assess fluid balance and detect electrolyte abnormalities.
Plasma osmolality measures the concentration of all dissolved particles in blood. It reflects the balance between water and solutes, and the body regulates it tightly between 275 and 295 mOsm/kg through thirst, vasopressin (ADH) release, and kidney function.
The standard clinical formula:
Posm = 2 x Na + (Glucose / 18) + (BUN / 2.8)
Where sodium (Na) is in mEq/L, glucose is in mg/dL, and BUN (blood urea nitrogen) is in mg/dL. The divisors convert glucose and BUN from mass units to milliosmoles.
Sodium is multiplied by 2 because it is accompanied by a roughly equal concentration of anions (mostly chloride and bicarbonate) to maintain electrical neutrality.
Normal range: 275-295 mOsm/kg.
Below 275 mOsm/kg (hypo-osmolality): The blood is more dilute than normal. Common causes include SIADH (syndrome of inappropriate antidiuretic hormone secretion), adrenal insufficiency, hypothyroidism, and excessive fluid intake. Hyponatremia is usually the driver.
Above 295 mOsm/kg (hyperosmolality): The blood is more concentrated. Dehydration is the most common cause. Hyperglycemia (as in diabetic ketoacidosis or hyperosmolar hyperglycemic state) and elevated BUN from kidney failure or high protein intake also contribute.
The osmolal gap: if a lab measures osmolality directly (by freezing-point depression) and the measured value exceeds the calculated value by more than 10 mOsm/kg, it suggests the presence of unmeasured osmoles — alcohols, methanol, ethylene glycol, or mannitol. This gap is a useful clinical clue in poisoning or unexplained altered mental status.
This calculator uses the standard Worthley formula. For educational purposes only — clinical decisions require lab-confirmed values and physician interpretation.
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