Hydrogen dispersion in an enclosure is numerically studied using simple analytical solutions and alarge-eddy-simulation based CFD code. In simple calculations, the interface height and temperaturerise of the upper layer are obtained based on mass and energy conservation and the centrelinehydrogen volume fraction is derived from similarity solutions of buoyant jets. The calculatedcentreline hydrogen volume fraction using the two methods agree with each other; however,discrepancies are found for the calculated total flammable volume as a result of the inability of simplecalculations in taking into account local mixing and diffusion. The CFD model, in contrast, is found tobe capable of correctly reproducing the diffusion and stratification phenomena during the mixingstage.