This research aims to develop a website-based e-learning for social arithmetic for Grade VII of junior high school, named “In Math”. This study refers to the Borg and Gall model with five stages. The first, the need for analysis for teachers and students. The second, planning and developing the initial product. The third, validation from material and language experts and media experts, and revising the product. The fourth, small group testing, while the fifth stage is large group testing on teachers and seventh grade students in a junior high school in Jakarta, Indonesia. Based on this research, the percentage of the product feasibility according to the material and language experts is 85.42%, 83.39% for media experts, 75% for the small group tested on teachers 75%, 86.67% for the evaluation of 21 students, 95.2% for the large group tested of teachers, and 86.25% for the evaluation of 41 students. The average percentage of the product feasibility at each stage reached 85.32% and is categorized as very feasible. Thus, the “In Math” website-based e-learning is feasible to use.