This research aims to develop a reliable and valid scale to determine middle school students' self-efficacy about estimation skills. In addition, with the developed scale, the estimation skill self-efficacy of middle school students was examined in terms of various variables. For these purposes, a draft scale of 40 items was developed by reviewing the literature and taking expert opinions. In this context, data were obtained from two different study groups in the research. The first stage of this research was the development of the scale, and the data obtained from 327 middle school students. The second stage of this research was the testing process of the developed scale, and it was carried out on the data obtained from 317 middle school students. While developing the scale, confirmatory factor analysis, exploratory factor analysis, Guttman Split-half values reliability and Cronbach Alpha internal consistency coefficient calculations were performed. As a result of the analysis, the total variance percentage of the scale consisting of 23 items and 5 factors was found to be 55.61%. In addition, it was seen that the model obtained as a result of CFA was at an acceptable level. Cronbach Alpha value for the whole scale was determined as 0.91. As a result of this study, a reliable and valid scale was developed to determine the middle-school students' self-efficacy for estimation skills. It was concluded that middle school students' self-efficacy for estimation skills differed according to grade level, but not according to gender. When total score of the whole scale was considered, it was seen that estimation skill self-efficacies of the female students were higher than of the male students but that difference was not significant. Besides, when the estimation skill self-efficacy levels of students were investigated according to their grade level in this research, it was concluded that the self-efficacy scores of the 7th grade students were higher than of the 5th, 6th and 8th grade students.