We have studied the magnetic interference of the critical currents of synthetic planar thin‐film grain‐boundary Josephson junctions. We find that the effective area of these junctions scales as the square of the widthwin contrast to the usualw(2&lgr;+d) dependence of sandwich‐type Josephson junctions. This behavior is a simple consequence of the magnetic response of thin‐film superconductors to perpendicular applied fields. A model based on the London theory yields the observed behavior. In addition, we find the correction to the interference pattern due to the effect of the corners.