Serum Osmolality Calculator

Serum osmolality measures the concentration of solutes dissolved in blood plasma, expressed in mOsm/kg. It reflects the body's fluid and electrolyte balance. Abnormal values can indicate dehydration, kidney disease, poisoning, or metabolic disorders, making it a critical lab parameter in acute care settings.

The standard formula is: Osmolality = 2 × Na + Glucose/18 + BUN/2.8, where sodium (Na) is in mEq/L, glucose is in mg/dL, and BUN is in mg/dL. Multiplying sodium by 2 accounts for accompanying anions. Dividing glucose by 18 and BUN by 2.8 converts them from mg/dL to mOsm/kg.

The normal range for serum osmolality is 285–295 mOsm/kg. Values below 285 mOsm/kg indicate hypo-osmolality (hyponatremia is a common cause), while values above 295 mOsm/kg indicate hyperosmolality, which may result from dehydration, hyperglycemia, or accumulation of solutes like urea or toxins.

The osmolal gap is the difference between the measured osmolality (from the lab) and the predicted osmolality (from the formula). A gap greater than 10 mOsm/kg is considered significant and may suggest the presence of unmeasured osmoles such as ethanol, methanol, ethylene glycol, mannitol, or other toxins in the bloodstream.

Common causes of an elevated osmolal gap include ingestion of toxic alcohols (ethanol, methanol, ethylene glycol, isopropanol), mannitol administration, and propylene glycol toxicity (e.g., from high-dose lorazepam infusions). In the ICU, an unexplained osmolal gap should prompt evaluation for toxic alcohol ingestion.

Yes. An elevated serum osmolality (above 295 mOsm/kg) in the context of high sodium levels often indicates dehydration or water deficit. Clinicians use it alongside urine osmolality and clinical findings to guide fluid resuscitation and monitor the effectiveness of rehydration therapy.

Propylene glycol, used as a solvent in medications like intravenous lorazepam, is an osmotically active molecule. At high doses or during continuous infusions, it can accumulate and significantly increase serum osmolality, sometimes causing hyperosmolarity and hyperosmolar coma — particularly in critically ill patients.

This calculator uses the conventional US unit system: sodium in mEq/L, glucose in mg/dL, and BUN in mg/dL. If your values are in mmol/L, convert glucose by multiplying by 18 and BUN by multiplying by 2.8 before entering them, or use a version of the formula that omits the divisors.