Currently, several calculation tools exist to estimate the contact stiffness in bolted joints. Unfortunately, the reliability of these methods is debated, or the methods are applicable only under specific contact interface parameters. Consequently, the evaluation of contact stiffness in bolted joints remains a challenge in practical applications. This research aims to investigate the validity and reliability of the existing calculation tools. To assess the calculation tools, a study of contact mechanics and the theory of bolted joints has been carried out and models were developed to verify the mathematical implementation of the theoretical background. Experimental measurements were performed and corresponding models were developed, through which the reliability of the existing contact stiffness methods was researched. Subsequently, new modelling strategies were developed to determine the contact stiffness in bolted joints, and their effectiveness was compared against the existing calculation tools and experimental measurements. Based on these comparisons, a newly developed method is proposed as a suitable option to determine the contact stiffness in bolted joints.