Osmolality, Calculated Unit Converter

Calculated osmolality is an estimate of the total concentration of dissolved particles in your blood, derived from a formula rather than measured directly by an instrument. The standard formula is 2(Na) + Glucose/18 + BUN/2.8, using your sodium, blood glucose, and blood urea nitrogen values.

Because sodium is the dominant solute in blood, it accounts for the largest share of the calculation. Glucose and urea make smaller but clinically meaningful contributions.

The normal reference range for serum osmolality is approximately 275–295 mOsm/kg. When compared with measured osmolality, the difference (called the osmol gap) can reveal the presence of unmeasured substances such as toxic alcohols.

Calculated osmolality is derived from three routine blood tests: sodium (regulated by the kidneys and adrenal glands), glucose (regulated by the pancreas and liver), and blood urea nitrogen (a byproduct of protein metabolism cleared by the kidneys).