This research aims to develop an interface design assessment instrument in the form of a web-based online questionnaire application utilizing Shneiderman's Eight Golden Rules of Interface Design principles as its theoretical basis. The development of questionnaire questions was carried out by evaluating the questionnaire that had been designed qualitatively through content validity interviews with UI/UX practitioners or experts as well as face validity interviews with target application users. The User Centered Design approach is used for the development of said application’s interface design while system development is then carried out using the Kanban approach method. The developed application is then implemented by assessing e-Learning and Online Travel Agent case study applications, resulting in a total of 339 respondents. Assessment results show that applications in the Online Travel Agent category would generally obtain higher scores compared to e-Learning applications. In addition, a series of interface design recommendations for several applications in each category were obtained from score and comparison analysis based on aspects with the lowest scores. Hence, it can be concluded that the implementation of the Rule8 questionnaire application as an interface design assessment instrument platform can be used as a benchmark for interface design performance as well as a tool for analyzing weak points in an application's interface design.