To effectively calculate the stress intensity factors in welded components, a weld toe magnification factor is introduced here that will allow for the influence of geometrical effects. This factor was derived from the data obtained in a parametric study performed by FEM. Several sets of weld toe magnification factor solutions have been presented, but these are applicable only to non-load-carrying cruciform or T-butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints such as cruciform, cover plate and longitudinal stiffener joints, the currently used weld toe magnification factors do not adequately describe the behavior of weld toe cracks. In the present study, the weld toe magnification factor solutions with exponents and fractions that have polynomial functions in terms of a/c and a/t - i.e., crack depths normalized by corresponding crack lengths and specimen thickness - were developed through a parametric study using 3-dimensional finite elements for the above mentioned three types of welded joints. These weld toe magnification factor solutions showed a tendency to increase at a/t < 0.2. Meanwhile, for 0.2 < a/t < 0.7, the effect of the attachment on the weld toe magnification factor decreased asymptotically. When the a/t ratio exceeded this range, the weld toe magnification factor became almost one of unity. The fatigue crack propagation life was evaluated by using the proposed weld toe magnification factor and by considering the propagation mechanisms of multiple-surface cracks, and it showed good agreement-to within a deviation factor of 2-between the experimental and calculated results for the fatigue crack propagation life for all welded joint.