In this paper, finite element (FE) analyses were performed to update and also extend the applicable ranges of the existing KI and crack-opening displacement (COD) solutions for nonidealized through-wall cracks (TWCs). Then, a surface to TWC transition model was proposed based on these solutions. The proposed model provides a criterion which determines when the final surface crack (SC) should transition to a TWC. It also provides a criterion to determine the two crack lengths (at the inner diameter (ID) and outer diameter (OD) surfaces) of the initial nonidealized TWC. Furthermore, crack growth of nonidealized TWCs can be simulated by using the proposed method. Finally, the proposed model was verified by demonstrating that it can well predict the surface to through-wall transition behavior when compared to the natural crack growth simulations.