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Calculated Serum Osmolality

Calculated serum osmolality = 2 × Na + glucose/18 + BUN/2.8 (mg/dL), or in SI units 2 × Na + glucose + urea (mmol/L). Normal is about 275–295 mOsm/kg.

  • Serum sodium (mmol/L or mEq/L)
  • Serum glucose (mg/dL or mmol/L)
  • BUN / serum urea (mg/dL or mmol/L)
—

Interpretation

ResultMeaning
< 275Hypo-osmolar
275–295Normal
296–320Hyperosmolar
> 320Severe (HHS range)

Formula

Calc Osm = 2 × Na⁺ + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. SI: 2 × Na⁺ + glucose (mmol/L) + urea (mmol/L). Effective osmolality = 2 × Na⁺ + glucose/18.

Clinical pearls

  • Urea crosses cell membranes freely, so effective osmolality (tonicity) excludes it; HHS is defined using effective osmolality ≥320.
  • Compare with measured osmolality to get the osmolal gap (toxic alcohols).

Examples

serum osmolality na 140 glucose 90 bun 14calc osm na 150 glucose 40 mmol/L urea 20 mmol/Lcalculated osmolality sodium 135 glucose 5 urea 6

FAQ

What is a normal serum osmolality?

About 275–295 mOsm/kg.

How do I calculate osmolality in SI units?

2 × sodium + glucose + urea, all in mmol/L.

Evidence

  1. Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations. Clin Chem 1975;21:190–4.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology