Specific surface area has been obtained by using air adsorption method which is simpler than other conventional method. Physical constants are obtained from standard materials having known specific surface area, and specific surface areas of unknown samples are calculated by using the physical constants. It has been observed that the larger surface area difference between standard and unknown samples resulted in the lower measurements accuracy. Since the measurements accuracy decreased linearly with the specific surface area difference between unknown and standard samples, a correction curve could be obtained. Reliability of specific surface area investigated in this study stringly depended on the physical constants. If the surface structure of unknown material is different from that of standard material, new physical constant should be recalculated. The air adsorption method was, however, effective for the material that has same or similar surface structure as that of standard material.