Serum Osmolality Calculator
Serum osmolality is a measure of solute concentration in plasma. The calculated (estimated) osmolality uses three key values from the basic metabolic panel: sodium (the dominant cation), glucose (which contributes osmoles in proportion to its concentration), and blood urea nitrogen (BUN, which is an effective osmole but freely crosses cell membranes). The formula is: Estimated Osm = 2 x Na + glucose/18 + BUN/2.8, yielding mOsm/kg. Normal is 275-295 mOsm/kg. Comparing this estimate to the directly measured osmolality (by freezing point depression) yields the osmol gap, which identifies unmeasured osmoles such as toxic alcohols. Enter values in standard US units (mg/dL for glucose and BUN).
Serum osmolality formula
Estimated Osm (mOsm/kg) = 2 x Na + glucose/18 + BUN/2.8
Osmol Gap = Measured Osm - Estimated Osm
Na in mEq/L; glucose and BUN in mg/dL. Dividing glucose by 18 converts mg/dL to mmol/L. Dividing BUN by 2.8 converts to mmol/L (BUN molecular weight 28, but reported as mg of nitrogen per dL). Normal osmol gap is below 10 mOsm/kg.
Interpretation
- Estimated Osm below 275: Hypo-osmolality. Usually reflects hyponatremia.
- Estimated Osm 275-295: Normal range.
- Estimated Osm above 295: Hyperosmolality. Investigate sodium, glucose, renal function, and potential toxins.
- Osmol gap above 10: Consider toxic alcohol ingestion (methanol, ethylene glycol), ethanol, mannitol, or propylene glycol.
Frequently asked questions
What is serum osmolality?
Serum osmolality measures the concentration of all osmotically active particles in serum, expressed in milliosmoles per kilogram (mOsm/kg). Normal is 275-295 mOsm/kg. It reflects the body's water balance and is regulated by ADH (antidiuretic hormone) and thirst.
What is the formula for estimated serum osmolality?
Estimated Osm = 2 x Na + glucose/18 + BUN/2.8. Sodium (mEq/L) is doubled to account for accompanying anions. Glucose (mg/dL) is divided by 18 to convert to mmol/L. BUN (mg/dL) is divided by 2.8 to convert to mmol/L. Units for the result: mOsm/kg.
What is the osmol gap?
The osmol gap = measured osmolality (by freezing point depression) minus calculated osmolality. A normal osmol gap is less than 10 mOsm/kg. An elevated gap (above 10) suggests the presence of unmeasured osmoles, such as ethanol, methanol, ethylene glycol, isopropanol, propylene glycol, or mannitol.
How does hyperglycemia affect osmolality?
Each 100 mg/dL rise in blood glucose adds approximately 5.5 mOsm/kg to measured osmolality. Severe hyperglycemia (as in hyperosmolar hyperglycemic state) can raise osmolality to 320-380 mOsm/kg, leading to cellular dehydration and altered mental status.
What causes hyperosmolality vs hypo-osmolality?
Hyperosmolality (above 295 mOsm/kg): Hypernatremia, hyperglycemia, azotemia, toxic alcohol ingestion. Hypo-osmolality (below 275 mOsm/kg): Hyponatremia is the most common cause; may reflect dilutional states (SIADH, heart failure, cirrhosis) or sodium depletion.
Official sources
- NIH MedlinePlus: Osmolality Tests.
- CDC: Chemical Emergencies and Toxic Exposures.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.