Nurses need to be able to make drug calculations competently. In this study, involving 229 second year British nursing students, we explored the influence of mathematics anxiety, self-efficacy, and numerical ability on drug calculation ability and determined which factors would best predict this skill. Strong significant relationships (p?.001) existed between anxiety, self-efficacy, and ability. Students who failed the numerical and/or drug calculation ability tests were more anxious (p?.001) and less confident (p?=?.002) in performing calculations than those who passed. Numerical ability made the strongest unique contribution in predicting drug calculation ability (beta?=?0.50, p?.001) followed by drug calculation self-efficacy (beta?=?0.16, p?=?.04). Early testing is recommended for basic numerical skills. Faculty are advised to refresh students' numerical skills before introducing drug calculations.