Estimated Serum Osmolality Calculator

Estimate serum osmolality from sodium, glucose and BUN.

Aug 31, 2026 Fluids, Electrolytes & Acid Base #sodium #electrolytes #osmolality

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All fields required unless noted

Serum sodium (mEq/L)

Serum glucose (mg/dL)

BUN (mg/dL)

Calculating...

Enter your values and press Calculate to see the result.

Formula & Clinical Notes

Formula

Osm = 2 × Na + Glucose / 18 + BUN / 2.8

Clinical Notes

The formula 2 × Na + glucose/18 + BUN/2.8 (glucose and BUN in mg/dL) estimates serum osmolality. The normal serum osmolality is approximately 275-295 mOsm/kg. A large difference between measured and estimated osmolality (the osmolal gap) suggests unmeasured osmoles such as ethanol, methanol or ethylene glycol.

Sources

StatPearls — Serum Osmolality
StatPearls — Osmometer