Clinician numeracy (CN), the ability to use and understand quantitative data in patient care, is an important skill for healthcare professionals. Nonetheless, it is recognized that many healthcare professionals, including doctors, have deficiencies in CN, and that this may affect patient safety. In our previous research using the Medical Interpretation and Numeracy Test (MINT), we found that many doctors in training in the UK had low CN. However, participants were not permitted to use calculators when taking the MINT, even though staff has access to calculators in clinical practice. Therefore, our original study may have underestimated doctors' CN, compared to their ability in clinical practice. We designed a randomized controlled trial to assess the impact of calculators on MINT score. We recruited 110 third-year medical students to participate in the study. Our results show that having access to a calculator had no impact on test scores. We consider that this is due to two factors: (1) CN is a complex construct that involves problem-solving and analysis, skills that are not improved by using calculators; (2) errors made by participants in our study are predominantly errors of understanding rather than calculation errors. We suggest that participants taking CN tests should have access to calculators as they would do in the workplace. We recognize that further research is needed, but suggest that educational interventions to improve CN should primarily be directed at improving understanding of mathematical concepts rather than focusing on calculation skills.