Hydrogen is a promising energy carrier that will become competitive in the near future. The present study modeled hydrogen leaks and diffusion in an actual size underground parking garage with the numerical model validated by scale experimental data. The results show that the hydrogen concentration distributions are not uniform in the gas mixture layer along the ceiling and the initial front velocity of the gas mixture layer decays with horizontal distance from the leaking car. The vertical filling front velocity for times after 600 s remain constant in the near field but increases linearly with distance in the far field. The corner walls did not significantly affect the far-field concentration distributions and the ventilation layout with vents in the garage corners provided better hydrogen removal. These results can be used to predict the hydrogen concentration buildup in large confined spaces and to help design underground parking garage ventilation systems. (c) 2021 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.