Serum Osmolality Calculator

Normal serum osmolality is 275 to 295 mOsm/kg. Values below 275 suggest hypo-osmolality (often from hyponatremia), while values above 295 suggest hyperosmolality (from dehydration, DKA, or HHS).

The osmolar gap is the difference between the measured osmolality (from a lab test) and this calculated value. A normal gap is less than 10 mOsm/kg. A gap above 10 to 12 suggests unmeasured osmoles like ethanol, methanol, or ethylene glycol.

Yes. Ethanol significantly increases measured osmolality, creating a large osmolar gap. Each 100 mg/dL of ethanol contributes approximately 22 mOsm/kg to the measured value.

It is most useful in evaluating suspected toxic alcohol ingestions (methanol, ethylene glycol), assessing hydration status, and evaluating the cause of hyponatremia or altered mental status.

Osmolality is measured per kilogram of solvent (mOsm/kg), while osmolarity is per liter of solution (mOsm/L). In clinical practice, the values are nearly identical and often used interchangeably.

No. This tool is for educational purposes only. Always consult a healthcare professional for interpretation of osmolality results, especially in emergency or toxicology settings.

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