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Calculator: Serum osmolal gap (SI units)*

Calculator: Serum osmolal gap (SI units)*

Serum Na   mmol/L
Serum glucose  
Serum BUN  
Serum ethanol  
Measured Osm  

Important: Inputs must be complete to perform calculation.

Calculated Osm  
Osmolal gap  
Decimal precision  


  • BUN: blood urea nitrogen; Osm: osmolality.
  • The plasma osmolality is determined by the concentration of the different solutes in the plasma.
  • In normal subjects, sodium salts (chloride and bicarbonate), glucose, and urea are the primary circulating solutes.
  • The calculated plasma osmolality normally is between 270 and 290 mOsm/kg H2O and the Osmolal gap is normally <10 mOsm/kg H2O. You can calculate the Osmolal gap from the formulas depicted.
  • * This calculator uses SI units (Système international d'unités) as the default for inputs. Do not use this calculator if you are using conventional units (also known as US units and gravimetric units); please refer instead to the conventional units version of this calculator.

Equations used
Calculated_Osm = (Serum_Na * 2) + Serum_glucose + Serum_BUN + (Serum_ethanol * 1.25)
Osmolal_gap = Measured_Osm - Calculated_Osm


Only digits 0 to 9 and a single decimal point (".") are acceptable as numeric inputs. Attempted input of other characters into a numeric field may lead to an incorrect result.

Information on this page may not appear correctly if printed.

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