Despite its complexity in science, sinking and floating is a phenomenon about which students of almost all grades develop personal theories, using a variety of conceptual elements such as weight, volume, shape, and density, prior to classroom teaching. Here, we distribute students from elementary to high school according to the levels of their achievement on the learning progression regarding the buoyancy phenomenon. We suggest four levels of learning progression for buoyancy concept. We developed a 13-item, open-ended, and short essay type questionnaire to evaluate students’ learning progression of buoyancy concept based on two different contexts. We evaluated the validity and reliability of the new instrument for measuring buoyancy learning progression using a series of rigorous statistical tests. Participants of this study were students in grades 3–12 (N = 1,017). A series of analyses including internal consistency analysis (Cronbach alpha), confirmatory factor analysis using structural equation modeling, and Rasch analysis for item fit and person/item reliability revealed that the instrument met the quality benchmark. Furthermore, our findings revealed that students’ abilities in two different contexts were differentiated. Finally, we discuss the four levels of learning progression, concept assessment items developed, and some implications based on the findings.