Serum Osmolality / Osmolarity Calculator

What are osmolality, osmolarity, and the osmolal gap?

Serum osmolality (mOsm/kg) is measured by freezing point depression osmometry. Osmolarity (mOsm/L) is a volume-based approximation. Clinically, osmolality is preferred.

Calculated osmolality estimates measured osmolality from sodium, glucose, and urea. The osmolal gap = measured − calculated and screens for unmeasured osmoles (e.g., toxic alcohols).

Formulas and unit handling

Dorwart–Chalmers (most accurate vs measured): 1.86×Na + Glu/18 + BUN/2.8 + 9. Add ethanol term if present: + Ethanol/3.7 (or +1.25×ethanol when in mmol/L).

Simple formula: 2×Na + Glu/18 + BUN/2.8. Effective osmolality (tonicity) excludes urea: 2×Na + Glu/18.

Conversions: Glucose mg/dL = 18×mmol/L. BUN mg/dL = 2.801×Urea mmol/L. Ethanol contribution ≈ mg/dL ÷ 3.7, or 1.25×mmol/L.

Reference ranges and cutoffs

• Normal serum osmolality: ~275–295 mOsm/kg.
• Normal osmolal gap: typically <10 mOsm/kg; many labs flag >15; ≥20 raises concern for toxic alcohols.
• Effective osmolality guides water shifts and tonicity.

Limitations

• Equations are approximations; compare with measured osmolality when available.
• Large ethanol levels inflate the gap; add ethanol term for better agreement.
• Severe hyperlipidemia/proteinemia can artifactually lower measured sodium (pseudohyponatremia).
• Use clinical context; abnormal gap alone is not diagnostic.