The COVID-19 pandemic has led to the closure of traditional fitness venues like gyms, while also leading to a significant increase in our health awareness. This circumstance inspired the development of ON-Fit, a fitness web application. This paper explores the design blueprint, which is structured around two fundamental components: the frontend and the backend architecture. The frontend encapsulates the user interface, prioritizing intuitive navigation, user experience, and seamless interaction. Meanwhile, the backend focuses on the technical framework, emphasizing data management, security protocols, and algorithmic functionality. This thesis discusses the interconnectedness between these two facets, shedding light on their importance in constructing a practical and influential fitness application.