In this paper, the influence of magnetic shunt geometry on the transformer leakage field and short-circuit impedance is examined. The magnetic-field computation is conducted with the use of a particular hybrid finite-element method (FEM) boundary-element method (BEM) formulation, facilitating the parametric investigation of magnetic shunt effects through application of appropriate boundary conditions. A design sensitivity analysis for the optimization of the shunt geometry ensuring a desired change in short-circuit impedance and shunt losses has also been developed, in conjunction with the magnetic-field model. Index Terms-Design optimization, finite-element (FEM) boundary-element (BEM) hybrid methods, sensitivity analysis, transformers.