This study presents a universal correlation for blast wave decay after hydrogen tank rupture in a tunnel fire. The validated CFD model is applied to perform numerical experiments in tunnels of cross-section area 24-139 m(2), aspect ratio width-height 1.2-2.7, tunnel length 150-1500 m with tanks of volume 15-176 L, and pressure 35-95 MPa (mass 0.6-6.9 kg). A dimensionless correlation for transition distance from Zone 1, dominated by blast wave reflections, to Zone 2 of planar wave propagation is developed. The traditional models derived using high explosives are found to be non-appropriate to describe blast wave decay after hydrogen tank rupture in a tunnel fire. Therefore, an original correlation is developed using methods of similitude analysis and numerical experiments. The mechanical and fraction of chemical energy contributing to the blast wave strength are accounted for along with effects of tunnel aspect ratio and friction/minor losses. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.