The experimental data on the hydrogen flame length normalized by the nozzle diameter are correlated with the dimensionless product of the density ratio (hydrogen density in the nozzle exit to the density of surrounding air) and the Mach number to the power of three. The current up-to-date experimental data on hydrogen flame length are used to build the correlation that covers laminar and turbulent flows, buoyancyand momentum-dominated releases, subsonic, sonic and highly under-expanded supersonic jets. The density and velocity of hydrogen in the nozzle are taken either directly from experiments or calculated by the under-expanded jet theory published elsewhere. The correlation is validated in the range of hydrogen storage pressures from nearly atmospheric up to 90 MPa and nozzle diameters from 0.4 to 51.7 mm. The predictive capability of this dimensionless correlation exceeds that of previously published work based on the Froude number only.KEYWORDS: hydrogen jet fire, flame length, under-expanded jet. NOMENCLATURE LISTING