Although planar solid oxide fuel cell and electrolyser technology is a key perspective for the next energy systems, it still suffers from a lack of efficient tightness solutions due to the need for the use of a mix of brittle ceramics and stiff metallic materials at high temperatures. In order to design new well adapted metallic seals, an original computational model is proposed. It links the evolution of the local mechanical fields to the leakage rate. It remains purely macroscopic and does not require a fine description of roughness. As the model is designed to deal with high temperature systems, it takes into account the strain rate dependence of the seal materials. High temperature leakage tests are realized under load control conditions using a seal consisting of a 0.3 mm thick Fecralloy sheet lying between two elastic bearings made of Udimet 720 nickel alloy. One of the bearings presents a boss for which several geometries are used. Finite element calculations are performed to describe the mechanical state of the seal as a function of time. These results are post-processed using the proposed model to derive an estimation of the leakage rate. The model is tuned against the experimental results. Finally the validity of the model is checked by comparing its predictions to additional experimental results in which seal geometry, loading history, gas pressure or gas composition are varied. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.