Abstract The test specimen size effect of fracture toughness Jc of the material in the ductile-to-brittle transition temperature (DBTT) region is known to not be ignored. One practical method was to use the 1-inch thickness (1T) compact tension (CT) test specimen to integrity assessment of cracked structure. To support this practice, a method to convert Jc obtained with other sized CT specimens is provided in ASTM E1921. On the other hand, a large number of fracture toughness test data have been collected for the single-edge notched bend (SE(B)) specimens. In order to practically use the SE(B) test data in engineering application, it is essential for converting Jc obtained from different sized SE(B) specimen to the 1T CT specimen Jc. In this paper, an engineering framework is proposed to convert minimum Jc obtained from various sized SE(B) specimens to the minimum 1T CT specimen Jc. The approach applies the modified Ritchie-Knott-Rice failure criterion, which predicts the onset of cleavage fracture when the crack-opening σ22 measured at a distance from the crack-tip equal to four times the crack-tip opening displacement σt, denoted as σ22d, exceeds a critical value σ22c, to the elastic-plastic finite element results. In addition, this framework utilizes our recent finding that the J when σ22d reaches the critical value σ22c has a possibility to correspond to the minimum Jc for a specified specimen configuration and material. The proposed framework has an advantage on the point that it requires only stress-strain curve as an experimental data. The validity of the framework was demonstrated through the experimental results.