We present an accurate method of obtaining approximate field components for a two-dimensional perpendicular magnetic recording head in the presence of a soft magnetic underlayer. It is easy to use and may be applied to any configuration of rectangular poles, shields, and gaps. Formulas for the potential at the air-bearing surface across a pole-shield gap or beyond a pole edge are obtained by assuming nonlinear magnetic potential drops both from a pole corner and from a shield corner to the underlayer. The fast Fourier transform then gives the potential and field components elsewhere. We demonstrate the accuracy of the field component estimates for an unshielded single pole and for a symmetrically shielded single pole.