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Serum Osmolality Calculator
Serum Osmolality measures the concentration of particles dissolved in the blood serum. It is a critical parameter for evaluating the body’s water balance and investigating hyponatremia.
This calculator determines the calculated osmolality based on the three primary solutes: Sodium, Glucose, and Urea (BUN). Comparison with measured osmolality (from the lab) allows for the detection of an Osmolal Gap, which may indicate the presence of foreign substances like methanol or ethylene glycol.
Laboratory Values
InputsmEq/L or mmol/L
mg/dL
mg/dL
Result
Normal Range: 275 – 295 mOsm/kg
Clinical Context
Formula (US Units):
Osmolality = (2 × Na) + (Glucose / 18) + (BUN / 2.8)
Osmolal Gap:
Osmolal Gap = Measured Osmolality − Calculated Osmolality.
A normal gap is ≤ 10 mOsm/kg. An elevated gap suggests the presence of unmeasured osmotically active particles such as:
• Toxic Alcohols: Methanol, Ethylene Glycol.
• Medications: Mannitol, Propylene Glycol (in IV infusions).
• Metabolic: Ketoacidosis (acetone).
Clinical References
How to Use This Calculator
Follow these steps to calculate Serum Osmolality.
Enter Sodium
Input serum Sodium in mEq/L (or mmol/L).
Enter Glucose & BUN
Input Glucose and BUN in mg/dL (standard US units).
Calculate
The calculated osmolality will be displayed. This value can be compared to the laboratory measured osmolality.
Frequently Asked Questions
If the patient has consumed alcohol, it contributes to the measured osmolality. You can estimate its contribution: Ethanol (mg/dL) / 4.6.
These factors convert mg/dL to mmol/L. Molecular weight of Glucose is ~180 (180/10=18). Molecular weight of Nitrogen in Urea is 28 (28/10=2.8).