The purposed of this paper is to investigates the required determination of gage length to calculate the elongation under various standard specifications for steel reinforcing bars, such as ISO 6892-1, JIS G 03112:2004, and ASTM E8M-11. In order to evaluate the elongations of the steel reinforcing bars, a series of tensile tests were carried out in the laboratory. Two different steel grades, namely Grades 420 and 500, were used in the study. From the test results, it can be concluded two main findings. The first finding is that the elongation in terms of strain in accordance with ASTM E8M-11 was found to be the lowest among all. The measurement according to the gage length of JIS 2201-1998 resulted the largest elongation values compared with those measured in accordance with the other two standard specifications. This indicates the gage length plays a very important role in defining the elongation of the steel bars under tension. Finally, Grade 550 is comparable to Grade 420 in terms of elongation. This means that Grade 550 is very promising to be used for longitudinal bars in special structural members, e.g. RC seismic-resistant elements.