This study presents a finite element (FE) investigation of stiffened and unstiffened box hollow columns having compact, non-compact and slender cross-sections for short as well as long columns. The available analytical methods neglected the effect of stiffener's length when calculating stiffened hollow steel sections axial capacity. Therefore, an extensive study was conducted on the effect of stiffener length on ultimate strength of steel columns having box hollow sections. Also, the effect of different numbers of stiffeners on ultimate strength of steel columns considering five different grades of steel was numerically studied using nonlinear finite element analysis. A nonlinear (FE) analysis of steel columns which accounts the effects of residual stresses and initial local and global imperfections in long columns was performed. The current FEM results and the analytical methods such as effective width equations were compared and discussed. The FE models built in this study is verified against the available experimental data under axial compression and showed good agreement. As a major result of the conducted analysis taking into account the studied parameters, a novel equation was proposed to predict the ultimate strength of box steel sections.