This study explored the relationship between accuracy of and confidence in performance of 114 prospective primary school teachers in answering diagnostic questions on potential difference in parallel electric circuits. The participants were required to indicate their confidence in their answers for each question. Bias and calibration indices were calculated for each prospective teacher. A resolution index was calculated for each of the 43 participants who had some variance in both their performance scores and confidence ratings. In this study, the participants were more likely to know when they were giving the correct answer than when they were giving an incorrect answer. The resolution of confidence was positively related to the calibration of confidence. The findings indicate that having good resolution of confidence may be a prerequisite to, but not sufficient for, being well calibrated. In general, the prospective teachers in this study were poorly calibrated in their understanding of the potential difference in parallel circuits, the main reason for this being overconfidence. We also compared the participants for gender differences and noted that female students were more underconfident and less overconfident than their male counterparts in their understanding of the potential difference in parallel circuits. Implications of the findings for teaching and future research are discussed.