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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 in power three. The all known up to date experimental data on hydrogen flame length are used to build the correlation that covers laminar and turbulent flows, buoyancy- and momentum-dominated releases, subsonic, sonic and highly underexpanded supersonic jets. Density and velocity of hydrogen in the nozzle are taken either directly from experiments or calculated by the underexpanded 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 and based on the Froude number only.