There are opposing views about calculator use in school mathematics. This paper reports on a study that investigated the arithmetic proficiency of mathematics 1 university students and the effects of calculator usage at school level on their proficiency. The study followed a descriptive survey design involving the use of questionnaire and data collected from students' examination scripts and a static-group comparison design to determine the effect of introducing calculators early in schooling. The results showed that students who started using calculators in primary school performed significantly worse compared to others who were exposed to calculators later in the schooling. In addition, students who habitually used calculators tend to show a lower arithmetic proficiency than those who sometimes use calculators. Based on the findings of this study, it is recommended that students should first master number computations before they are introduced to the use of calculators.