This paper investigates how graphene is doped by adsorption on metal substrates using density functional theory (DFT). The study shows that weak bonding on metals like Al, Ag, Cu, Au, and Pt can shift the Fermi level of graphene by ~0.5 eV relative to its conical points. The crossover from p-type to n-type doping occurs when the metal's work function is ~5.4 eV, which is much higher than graphene's work function of 4.5 eV. A simple analytical model is developed that uses only the work functions of the metal and graphene to predict the Fermi level shift and doping type. Graphene's unique electronic structure, characterized by zero band gap and linear dispersion near the Fermi energy, is significantly altered by strong adsorption on metals like Co, Ni, and Pd, but preserved by weak adsorption on other metals. The doping type (n-type or p-type) depends on the difference between the metal and graphene work functions. The results show that the Fermi level shifts when the metal-graphene interface potential step is considered. The study also reveals that the charge redistribution at the graphene-metal interface is not only due to electron transfer but also due to chemical interactions between the metal and graphene. These interactions play a significant role in forming an interface dipole and potential step. The model developed in this paper accounts for both the electron transfer and chemical interaction, and it is shown to accurately predict the Fermi level shifts for various metals. The model is based on the work functions of the metal and graphene and is applicable to all metal substrates. It is demonstrated that the model can predict the Fermi level shifts and the type of doping in graphene for different metals. The results show that the critical metal work function where the Fermi level coincides with the conical points of graphene is ~5.4 eV, which is much higher than the work function of graphene. This indicates that the chemical interaction between the metal and graphene is significant in determining the doping characteristics of graphene.This paper investigates how graphene is doped by adsorption on metal substrates using density functional theory (DFT). The study shows that weak bonding on metals like Al, Ag, Cu, Au, and Pt can shift the Fermi level of graphene by ~0.5 eV relative to its conical points. The crossover from p-type to n-type doping occurs when the metal's work function is ~5.4 eV, which is much higher than graphene's work function of 4.5 eV. A simple analytical model is developed that uses only the work functions of the metal and graphene to predict the Fermi level shift and doping type. Graphene's unique electronic structure, characterized by zero band gap and linear dispersion near the Fermi energy, is significantly altered by strong adsorption on metals like Co, Ni, and Pd, but preserved by weak adsorption on other metals. The doping type (n-type or p-type) depends on the difference between the metal and graphene work functions. The results show that the Fermi level shifts when the metal-graphene interface potential step is considered. The study also reveals that the charge redistribution at the graphene-metal interface is not only due to electron transfer but also due to chemical interactions between the metal and graphene. These interactions play a significant role in forming an interface dipole and potential step. The model developed in this paper accounts for both the electron transfer and chemical interaction, and it is shown to accurately predict the Fermi level shifts for various metals. The model is based on the work functions of the metal and graphene and is applicable to all metal substrates. It is demonstrated that the model can predict the Fermi level shifts and the type of doping in graphene for different metals. The results show that the critical metal work function where the Fermi level coincides with the conical points of graphene is ~5.4 eV, which is much higher than the work function of graphene. This indicates that the chemical interaction between the metal and graphene is significant in determining the doping characteristics of graphene.