The paper investigates the mechanical properties of structural steels, focusing on hardness and toughness, which are important indicators used in industrial applications. In total, four major hardness testing methods were analysed: Vickers, Rockwell, Brinell, and Leeb, and the Charpy impact test was used to measure toughness. The hardness tests evaluated the precision with which steel resistance to deformation can be determined under different conditions, and the Charpy test examined their energy absorption capacity in case of sudden fractures and tensile testing was estimated on them to study the range of yield strength and ultimate tensile strength. This research has conducted 288 experimental tests to evaluate the correlation between various hardness testing methods (Brinell, Vickers, Rockwell, and Leeb) and the mechanical properties of the structural steel samples. Further, 42 experimental tests were performed for the Charpy impact test to evaluate a correlation between Charpy impact values and mechanical properties of the structural steel samples. 330 samples were prepared for the respective tests, representing various structural steel grades (Grade 300, 350, and 400). Among different grades of steel samples, 350-grade steel exhibited a higher impact energy of 94.4 J. Both Charpy Impact Values and Hardness Test Values were plotted against ultimate tensile strength and yield strength to determine if there is a correlation between the properties. The results reveal a better accuracy of Vickers and Rockwell tests as they revealed only an error percentage of 2.5% and 1.9%, respectively, especially for fine-grained materials, while Leeb is characterized by good convenience in large-scale testing at the site. Although the Brinell test is very effective for coarse structures, it was not as precise as martensite in more complex microstructures. Both hardness and tensile strength showed a strong correlation, indicating that hardness may accurately predict the material strength. The study also highlights the reverse relationship between hardness and toughness. This research contributes to structural steel testing by proposing methods to predict mechanical properties using non-destructive testing techniques. It sets the stage for future improvementsin hardness testing models by incorporating factors like microstructural variations, strain hardening, and heat treatment, which can enhance the accuracy of predictions. Furthermore, the research underscores the importance of portable hardness testing methods and emphasizes the need for improved calibration protocols to ensure more accurate in-situ measurements.