The fatigue strength of cast components is significantly influenced by either interior inhomogenities, such as porosity or intermetallic phases, as well as the notch acuity of the as-cast surface layer. These imperfections act as geometric notches and cause a local stress concentration, which leads to a damage of the material by initiating a fatigue crack under cyclic loading. To examine the influence of such inhomogenities on the fatigue behavior, the application of the concept according to Murakami is proven to be feasible. Therefore, a stress intensity factor of a planar crack in shape of the 2D-projected defect area perpendicular to the load direction is evaluated. For the consideration of the defect shape and location, a dimensionless geometry factor is utilized, which has to take the defects geometry into account. Beside an accurate geometric description of the inhomogenities, a profound knowledge of the material behavior regarding crack closure mechanisms as well as crack growth is needed. The aim of this work is to improve the accuracy of fracture mechanical concepts on the characterization of casting defects. For the parametric study, numerical analysis utilizing the software ABAQUS and FRANC3D are carried out. In addition to common analytic solutions of geometry factors, more complex shape and location parameters are examined within the numerical studies. It is found that especially the distance to the surface has a major influence on the maximum stress intensity factor along the crack front. Hence, neglecting the defect location in respect to the surface may cause a non-conservative assessment of crack initiation. An additional part of the work is the analysis of crack growth data collected in cyclic tests of SENB-specimens. The variation of the length within roughness and oxide induced crack closure is fully build-up and reveals that an exclusion of such mechanism also leads to non-conservative lifetime assessment. Based on various numeric simulations on initial cracks with identical area, but different shape and location, the influence on the crack growth is evaluated. A final comparison of the results concludes that the defect contour not only influences the crack initiation but also the fatigue lifetime, validating that the numerically evaluated geometry factors in this work significantly contribute to an improved fracture propagation based fatigue design.