Arithmetic and the ability to use numbers are important skills. Numbers can be represented in three ways: through number words, Arabic symbols or non-symbolically. Much research attention has focused on how associations form between these three numerical representations. However, it is not yet clear whether these associations are automatic or if they require working memory (WM) resources. In this registered report, we used the dual-task paradigm to answer this question. Eighty-one adults were administered dot, digit and cross-modal (i.e. dot versus digit) magnitude comparison tasks in standalone and dual-task conditions with phonological (PL) or visuospatial (VSSP) WM interference. We found that all three types of magnitude comparison necessitated WM resources. Symbolic comparison necessitated VSSP WM. Surprisingly, in this task, accuracy improved under both WM interference conditions, evidencing the proposal that introducing executive function challenges in simple and familiar tasks can improve performance. In non-symbolic comparison, our findings demonstrated that the VSSP and the PL—albeit to a lesser extent—were employed. Finally, cross-modal comparison necessitated VSSP WM. These findings evidence the fundamental role visuospatial processing plays in numerical processing and that adults require WM resources for simply processing numerical representations and translating between them.