In resistance spot welding, which is widely used for assembling automobile bodies, reduction of weld spacing (short-pitch welding) has been put to practical use to improve the strength and stiffness of the welded assemblies and to advance collision safety and ride comfort. In short-pitch welding, shunt current going through the previously welded points is inevitable and delays nugget formation and growth, but it is difficult to know the degree of shunting because it varies depending not only on the weld spacing but also on the sheet combination. In order to study the shunt phenomenon, several theoretical models have been proposed, but they have some critical problems in practical use, such as overestimation of the shunt resistance and many parameters to be determined from experiments. In this study, a mathematical model is established which can easily predict the ratio of effective weld current (or non-effective shunt current) to the total input current with only one experimental parameter, in the case of two-sheet stack-up with a single existing (shunting) weld. By using this model, delayed nugget growth curve of the second weld can be predicted taking the shunting effects into account, from the known nugget growth curve of the first weld. Furthermore, the efficiency of the shunt current to ease some three-sheet stack-ups welding was shown experimentally, and the optimal weld-spacing to maximize the welding current range was estimated by the proposed model.