We study exchange couplings in the organic magnets copperII phthalocyanine CuIIPc and manga-neseII phthalocyanine MnIIPc by a combination of Green's function perturbation theory and ab initio density-functional theory DFT. Based on the indirect exchange model, our perturbation-theory calculation of CuIIPc qualitatively agrees with the experimental observations. DFT calculations performed on CuIIPc dimer show a very good quantitative agreement with exchange couplings that our theoretical group extracts by using a global fitting for the magnetization measurements to a spin-1 2 Bonner-Fisher model. These two methods give us remarkably consistent trends for the exchange couplings in CuIIPc when changing the stacking angles. The situation is more complex for MnIIPc owing to the competition between superexchange and indirect exchange.