Calculated Plasma Osmolality
Enter serum sodium, glucose, blood urea nitrogen, and optionally ethanol to estimate plasma osmolality using the standard formula.
Include ethanol when intoxication is suspected to refine osmolar gap interpretation.
Calculated osmolality
290.0 mOsm/kg
Excludes ethanol contribution
Calculated + ethanol
290.0 mOsm/kg
How to Use This Calculator
Collect laboratory values
Use serum sodium, glucose, and BUN from the same blood draw. Add ethanol if available to refine osmolar gap analysis.
Calculate plasma osmolality
The calculator applies the standard formula (2 × Na⁺ + Glucose/18 + BUN/2.8) and optionally adds ethanol/4.6.
Interpret alongside measured osmolality
Compare calculated to measured osmolality to derive the osmolar gap. Gaps >10–15 mOsm/kg suggest unmeasured osmoles.
Formula
Calculated osmolality (mOsm/kg) = 2 × Na⁺ + Glucose/18 + BUN/2.8
With ethanol = Calculated osmolality + Ethanol/4.6
All inputs in mg/dL. For SI units (mmol/L), convert appropriately or use the serum osmolality calculator.
Full Description
Calculated plasma osmolality approximates the concentration of solutes contributing to serum tonicity. It helps evaluate hyperosmolar states, guide sodium correction, and identify osmolar gaps suggestive of toxic alcohol ingestion. Always interpret alongside measured osmolality, acid-base status, and clinical presentation. Adjust for additional osmoles such as mannitol when relevant.
Frequently Asked Questions
What is a normal plasma osmolality?
Normal range is 275–295 mOsm/kg. Values outside this range suggest hypo- or hyperosmolar disorders.
How is the osmolar gap calculated?
Osmolar gap = Measured osmolality − Calculated osmolality. A gap >10–15 mOsm/kg indicates unmeasured osmoles (toxins, ketoacids).
Why add ethanol to the formula?
Ethanol significantly contributes to osmolality. Including ethanol refines the calculated value, reducing the apparent osmolar gap.
Can I use mmol/L inputs?
This calculator uses mg/dL. For SI units, convert (glucose mmol/L × 18, urea mmol/L × 2.8) or use the serum osmolality calculator with SI-ready inputs.