a b s t r a c t a r t i c l e i n f o In this study, three aims were addressed: (1) validating a visuospatial working memory task in Math Garden, an adaptive online tool in which item difficulty and person ability are determined on the fly, (2) investigating the contribution of different item characteristics to the difficulty of the visuospatial working memory items, and (3) investigating relations between visuospatial working memory and various math domains at different ages. The method was validated by showing that item ratings were stable and grade differences in ability were significant. Regression analyses on the item level showed that not only sequence length, but also other characteristics, such as type of task (forward or backward), explained variance in item difficulty. Finally, regression analyses on the child level showed that visuospatial working memory and mathematics were significantly related: especially for addition and subtraction in the lower grades. For multiplication and division this relationship was weaker and without age trend