The study examines how students' conceptual understanding changes from high confidence with incorrect conceptions to high confidence with correct conceptions when reasoning about electromagnetics. The Conceptual Survey of Electricity and Magnetism test is weighted with students' self-rated confidence on each item in order to infer how strongly conceptions are held. The data (N = 118) are collected from an undergraduate static field theory course at a technical university in Finland. The data are analyzed using frequency distributions, correlation, and independent-samples "t" test. In overall, students' confidence grows along with the conceptual gains after instruction. The significant difference in the development of conceptual understanding between high-achieving and low-achieving students is found in the conceptual area of Newton's laws. Several strongly held alternative conceptions are also identified.