Electrolytes and acid–base

Calculated serum osmolality

Estimate serum osmolality using sodium, glucose and blood urea nitrogen.

Intended population: Conventional calculation with sodium in mmol/L and glucose/BUN in mg/dL.

Calculated, not measured, osmolality; ethanol and other unmeasured osmoles are excluded. Not effective osmolality/tonicity, which excludes urea. A normal estimate does not exclude toxic alcohol exposure.

Sodium (mmol/L)

Glucose (mg/dL)

Blood urea nitrogen (BUN), not total urea (mg/dL)

Sources and scoring references

UCSF Hospital Handbook: osmolality formula ↗Rasouli and Kalantari: comparison of osmolality equations (2005) ↗

Implementation references reviewed October 28, 2026. Verify applicability and unusual or treatment-changing results against the linked references and local guidance.