Calculated Plasma Osmolality
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This is an estimate, not a directly measured freezing-point osmolality. Enter BUN as urea nitrogen in mg/dL, not a urea concentration.
Simplified Na + K equation
Measured serum osmolality reference interval: 275–295 mOsm/kg (Mayo Clinic Laboratories). Do not use this measured reference interval to classify a calculated estimate. Laboratory-specific intervals may differ.
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Formula, unit conversions, limitations, and sources
Equations
With K: 2 × (Na + K) + glucose / 18 + BUN / 2.8. Without K: 2 × Na + glucose / 18 + BUN / 2.8. Na and K are entered in mEq/L; glucose and BUN in mg/dL. BUN is urea nitrogen, not urea, and the terms are not interchangeable.
Hypothetical example
Na 140 mEq/L, K 4 mEq/L, BUN 14 mg/dL, glucose 90 mg/dL: Na + K estimate 298.0 mOsm/kg; Na-only estimate 290.0 mOsm/kg.
Applicability
The model estimates a subset of osmoles from supplied analytes. Other osmoles such as alcohols may contribute to a measured value. This calculator does not compute an osmolal gap or diagnose and manage osmolar disorders, nor interpret the measured-serum reference interval against the estimate.
Sources and review
Sources checked 2026-11-10.