Serumosmolality measured by cryoscopic techniquein laboratory is the reference method.In clinical settings,serum osmolality measurement is notfeasible at bedside. In normal subjects, sodium, potassium glucose, and urea are the primary circulating solutes. These solute concentrations can be used to predict measured osmolality if no other solutes present at high millimolar concentrations.Many equations of serum osmolality have been proposed. The osmolal gap (OG) is the difference between measured osmolality and calculated osmolality.The major use of OG is to screen for presence of exogenous toxic substances and to screen alcohol intoxicationcases.Aim/Objective: The purpose of this study was to compare the calculated osmolality using various formulaewith the measured osmolality to determine which calculated formula fit best with measured osmolality.Materials and Methods: Serum osmolality resultsfromJanuary 2015 to December 2015 were extracted from the laboratory information system (LIS). Serum osmolality performed simultaneously with renal and liver function tests, serum electrolytes and plasma glucose were included.Serum osmolality measured for patients with the history of drug abuse and poisoning were excludedfrom the study.405serum osmolality results were chosen and divided into two groups. Group 1 included 205data with normal serum osmolality, renal, liver function tests and plasma glucose level less than 7.8 mmol/L.For the second group (n=200), data with low serum osmolality (n=90) and high serum osmolality (n=80) and normal serum osmolality (n=30) were included. Group 1 data was to identify which equation correlated with the measured osmolality and the Group 2 datato study the performance of equationthat correlated with the measuredosmolality.Results: Only four out of19 formulae were identified as optimal by havingthemean OG ≤ 2 mOsm/kg. The Smithline-Gardner formula (2Na+ Glu + BUN) showed the smallest osmolal gap with mean bias 0.3mOsm/kg. The Dorwart-Chalmers formulaincorporated in most autoanalysers for calculation of osmolalityunderestimatedcompared to measured osmolality.Conclusion: We recommend Smithline-Gardner formula for calculation of osmolal gap, as the OG gap is close to zero, simple, easy to calculate at bedside and easily incorporated in the Laboratory Information System.