This study aims to develop a scale to identify the factors influencing mathematics teachers' grading practices regarding students' in-class performance. The study was carried out with 180 secondary and 140 high school mathematics teachers from the southwestern region of Türkiye. The scale's construct validity was determined using item analysis, exploratory factor analysis (EFA), and confirmatory factor analysis (CFA). The EFA results showed that the scale consisted of a 5-point Likert-type scale with 36 items under eight factors (mathematical knowledge and skills, social behaviors, affective skills, effort, homework performance, follow-up test results, academic exam results, and external benchmarks). The scale explained 70.02% of the total variance, with factor loadings ranging from 0.50 to 0.92. The item-total correlations ranged from 0.36 to 0.64, and t-test results for the item discrimination index were significant. The Cronbach's alpha coefficient for the overall scale was calculated to be 0.92. The CFA results indicated that the scale model had a good fit (Chi-square/df = 1.39; RMSEA = 0.051; IFI = 0.98; NNFI = 0.97; CFI= 0.98; and SRMR = 0.062). Based on the findings, the scale is a valid and reliable instrument that may be used in determining the factors influencing mathematics teachers' grading practices.