An experimental formula for estimating porosity in a metal hydride packed bed is presented. The formula was developed by direct observation of the volume changes of a metal hydride packed bed under free expansion in a vessel. The experimental results showed that the cycles of expansion and contraction were repeated at large porosities above 60% after a rapid state change caused by early particle breakup. The formula for porosity was expressed as a function of the reacted fraction and as a function of the cycle number. The function formula of the reacted fraction can be used to compute different values of porosity for expansion by absorption and for contraction by desorption. The coefficients assuming 100% hydrogen storage based on the experiments with LaNi5 were an expansion ratio of 16.7% and a contraction ratio of 8.4%, on average. This experimental porosity formula is useful for effective thermal conductivity calculations and for numerical simulations of metal hydride packed bed behavior. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.