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The study of the interaction of hydrogen with transition metal surfaces holds great importance in numerous technological applications such as hydrogen storage and catalysis. Among the promising catalysts, ruthenium (Ru) stands out for its role in hydrogenation and hydrogenolysis reactions. The atomic hydrogen coverage can play a critical role in determining the outcome of these reactions, but the exact atomic hydrogen coverage of the Ru surface under catalytic conditions remains unknown. To address this knowledge gap, we conducted a comprehensive investigation of hydrogen adsorption and dissociation on the Ru (0001) surface using dispersion-corrected density functional theory calculations. Our study reveals that ab initio thermodynamic surface phase diagrams, based on the direct dissociation of gas-phase H2, are unable to predict the coverage observed in vacuum scanning tunneling microscopy (STM) experiments, which typically show submonolayer coverages. This discrepancy is explained by the fact that above 0.75 monolayer (ML) coverage, H2adsorption becomes unfavorable due to the electrostatic repulsion between the adsorbed hydrogen atoms and the incoming gas-phase H2molecules. Additionally, the activation energy for adsorbed H2dissociation increases at higher coverages. Thus, to accurately predict the atomic hydrogen coverage under experimental conditions, we computed a kinetic phase diagram based on the comparison of the rates for adsorbed H2dissociation and desorption. This approach successfully predicts a maximum achievable atomic hydrogen coverage of 0.875 ML under vacuum STM conditions. Furthermore, our findings show that the surface is not fully saturated by hydrogen under typical catalytic conditions, thus allowing for the adsorption of molecules required for catalytic processes. Overall, this study shows that H2adsorption and dissociation kinetics must be considered for the accurate prediction of the atomic hydrogen coverage under experimental conditions.