This paper presents an analysis of computational fluid dynamic models of compressed hydrogen gas leaksinto the air under different conditions to determine the volume of the hydrogen/air mixture and the extents ofthe lower flammable limit. The necessary hole size was calculated to determine a reasonably expectedhydrogen leak rate from a valve or a fitting of 5 and 20 cfm under 400 bars, resulting in a 0.1 and 0.2 mmeffective diameter hole respectively. The results were compared to calculated hypothetical volumes fromIEC 60079-10 for the same mass flowrate and in most cases the CFD results produced significantly smallerhydrogen/air volumes than the IEC standard. Prescriptive electrical classification distances in existingstandards for hydrogen and compressed natural gas were examined but they do not consider storage pressureand there appears to be no scientific basis for the distance determination. A proposed table of electricalclassificat ion distances incorporating hydrogen storage volume and pressure was produced based on thehydrogen LFL extents from a 0.2 mm diameter hole and the requirements of existing standards. ThePHOENICS CFD software package was used to solve the continuity, momentum and concentrationequations with the appropriate boundary conditions, buoyancy model and turbulence models. Numericalresults on hydrogen concentration predictions were obtained in the real industrial environment, typical for ahydrogen refueling or energy station.