The paper investigates the doping of graphene by adsorption on metal substrates using density functional theory (DFT). The authors find that weak bonding on metals such as Al, Ag, Cu, Au, and Pt can shift the Fermi level of graphene by approximately 0.5 eV, while preserving its unique electronic structure. At equilibrium separations, the crossover from p-type to n-type doping occurs for a metal work function of about 5.4 eV, significantly larger than the graphene work function of 4.5 eV. A simple analytical model is developed to describe the Fermi level shift in graphene, which is based solely on the work functions of the metal and graphene. This model accurately predicts the Fermi level shift and the type and concentration of charge carriers in graphene, extending the applicability of DFT results to practical device systems. The study also highlights the importance of both electron transfer and chemical interaction at the metal-graphene interface in determining the doping behavior.The paper investigates the doping of graphene by adsorption on metal substrates using density functional theory (DFT). The authors find that weak bonding on metals such as Al, Ag, Cu, Au, and Pt can shift the Fermi level of graphene by approximately 0.5 eV, while preserving its unique electronic structure. At equilibrium separations, the crossover from p-type to n-type doping occurs for a metal work function of about 5.4 eV, significantly larger than the graphene work function of 4.5 eV. A simple analytical model is developed to describe the Fermi level shift in graphene, which is based solely on the work functions of the metal and graphene. This model accurately predicts the Fermi level shift and the type and concentration of charge carriers in graphene, extending the applicability of DFT results to practical device systems. The study also highlights the importance of both electron transfer and chemical interaction at the metal-graphene interface in determining the doping behavior.