We present a new calculation of the diffuse gamma-ray emission from the Fanaroff-Riley I and II radio galaxies, exploiting a possible correlation between gamma and radio emission. The latter is studied for a set of radio galaxies resolved by the Fermi LAT telescope and is tested against gamma-ray upper limits for a number of radio loud AGN. We estimate the gamma-ray flux for the unresolved radio galaxy population which can contribute to the isotropic diffuse gamma-ray background.