Calculate estimated serum osmolality from sodium, glucose, and BUN or urea, with unit conversions and an optional measured osmolality gap.

Select the analyte and unit exactly as reported. BUN and urea are not the same mass concentration.

Optional settings

Fluid Osmolarity Formula

The following empirical formula estimates serum osmolality. It is not a general calculation of the osmolarity of an IV fluid.

O = (2 × Na) + (Glucose / 18) + (BUN / 2.8)

Variables:

  • O is the estimated serum osmolality (clinical convention: mOsm/kg)
  • Na is the sodium concentration (mmol/L)
  • Glucose is the glucose concentration (mg/dL)
  • BUN is the blood urea nitrogen concentration (mg/dL)

To estimate serum osmolality, multiply sodium in mmol/L by 2, add glucose in mg/dL divided by 18, and add BUN in mg/dL divided by 2.8. Glucose and urea already reported in mmol/L enter directly. For the osmolal gap, subtract this estimate from measured osmolality from the same specimen.

What is a Fluid Osmolarity?

Osmolarity is the concentration of solute particles per litre of solution; osmolality is expressed per kilogram of solvent. This calculator uses sodium, glucose and BUN or urea to estimate serum osmolality under a commonly used clinical formula. A result or osmolal gap alone does not diagnose hydration status, poisoning, kidney disease or an electrolyte disorder. Laboratory formulas differ, and this calculation excludes ethanol and other unmeasured osmoles. Interpret results with the laboratory method and a qualified clinician.