The purpose of this work was to explore how elementary pre-service' teachers (PSTs) responded to a volume task that asked for a coordination of changes in one dimension with changes in volume. We carried out both written pre-assessments and follow-up interviews with seventeen PSTs, focused on exploring volume content knowledge. Our findings indicated that the PSTs in the study used primarily the volume formula to respond to the task correctly. In follow-up interviews, PSTs struggled to justify their answers in different ways, however. Recommendations for supporting PSTs in similar teacher education programmes are discussed.