The objective of this paper is to ease the designing of mechanical spring, reduce calculation time, and improve accuracy and optimization in design using a Knowledge-based Engineering approach. The Application programming interface (API) was programmed using Java on the NetBeans IDE, which provides a variety of tools to build applications. Several standard reference books, research papers, and links were studied to finalize the formulae and factors like materials of the springs. The design for a user-friendly Graphic User Interface (GUI) was made considering the input and output values. To examine the API, we tested it with some numerical problem statements for each spring. From the results obtained, it was observed to have 0 to 3% of the error, when compared with manual calculations. The objectives of the calculator were achieved as the time of calculations was reduced by 95%. Other aims of accuracy and efficiency desired were fulfilled too. As a result of all this, the overall cost, errors, and complexities of mechanical components design are also reduced. The findings prove that this API can be a quick and efficient method to choose the spring of desired requirements for spring manufacturers and designers as it enhances design assistance. The API aids Design Automation and CAD Customization. The API calculator displays if the designed spring is safe and if a guide is required. The user can choose materials and can customize the calculator as required. © 2021 Author(s).