The development of calculation and estimation skills and the contribution of cognitive competencies such as memory to these skills are not well understood. The present study compared double-digit estimation and calculation in 60 children from grades three to eight. Presentation format (horizontal vs. vertical) and complexity (carry vs. no-carry) were manipulated for both types of math problems. The relations of these math skills to verbal and visual spatial memory were examined. Calculation and estimation showed grade related changes, but children were slower and less accurate at estimation than calculation across grades. Complexity affected problem solving accuracy. In general, visual spatial working memory was related to estimation. For calculation, verbal working memory was related to vertical carry problems, and verbal short term memory was related to no carry problems. Results are discussed with reference to theories about the development of numerical skills and their relations to memory.