Enabling comparable scores across grades is of interest for policymakers to evaluate educational systems, for researchers to investigate substantive questions, and for teachers to infer student growth. This study implemented a vertical scaling design to numeracy tests given in grades 5 and 8 as part of the Norwegian national testing system. Our design bridges the gap between grades 5 and 8 with a linking test tailored for grades 6 and 7, without the need for new item development. The design combines the existing administration for all grade 5 and 8 students with additional tests for samples of grade 6 and 7 students. The findings indicate that vertically scaling existing tests is possible through a cost-effective design and that numeracy, as measured by the Norwegian national tests, is comparable across four grades. We discuss the implications of our study for creating vertical scales in the context of national assessment systems.