This study presents the design results of a C-sharp based computer program developed for the design of laterally unrestrained I-section steel beams. The program was developed based on the stipulations of BS 5950 and Eurocode 3 (EC3) design standards. Several sets of steel beam models having the same cross-sectional dimensions but different laterally unrestrained span lengths were designed using the developed program, and the results were validated using an established software, Staad Pro. The design results obtained were found similar to the results obtained using Staad Pro. For a specific beam section with constant loadings, as the span length of the laterally unrestrained compression flange increases the buckling capacity reduces, thus the longer the beam, the more it is susceptible to lateral torsional buckling. Comparison of the results obtained using BS 5950 to those of EC 3 at different laterally unrestrained span lengths revealed that the areas of design sections obtained for BS 5950 are 21.5%, on the average, higher than those of EC3. Thus, beams with laterally unrestrained compression flange designed according to the requirements of EC 3 are more economical. The difference in results is because of the differences in the principles of design and measures used between the two standards.