Financial literacy is a person's skills regarding financial knowledge and behavior. In 2019, the National Survey of Financial Literacy and Inclusion (SNLIK) stated that Indonesia had a low level of financial literacy with a percentage of 38.03%. In 2016, the Indonesian government incorporated financial literacy into the K13 curriculum for the high school level to improve financial literacy at the student level. In line with that, students need facilities to help them learn financial literacy. This study aims to build an interaction design of financial mathematics calculators using the User-Centered Design method. The limitation of this study is the design uses a mobile web platform that targets high school students. The results of usability testing on effectiveness get an average task-completion rate score of 92.3%, an average efficiency of 92.0% overall relative efficiency, user satisfaction using the System Usability Scale (SUS) gets a score of 93.2 for the usability factor and a score of 74 for the learnability factor. The findings of this study indicate that the application is easy to use and provides a positive experience in learning financial literacy. The current study's results will support this application's use in future research as a new tool for providing financial literacy.