This paper presents a method for quick determination of plasma shape and position in a tokamak. It is based on relating the plasma shape parameters to multipoles of the external poloidal magnetic field along the midplane. The technique can therefore be used for time dependent studies of the plasma in an electromagnetic environment in lieu of requiring an MHD calculation at each time interval. We obtain simple curves relating the plasma shape parameters to the magnetic multipoles and use standard schemes for determining plasma major and minor radii. The method is compared with actual free boundary MHD calculations.