This paper presents a simple model to predict the flame speed and the blast pressure during an unconfined gas explosion. The proposed model is a modification to the fractal-based model proposed by Gostintsev et al. In the original model, the flame radius, r, is expressed as a function of time, t, as r/(kappa/epsilon S-L) = c(g)[t/(kappa/epsilon S-2(L)2)](alpha), where kappa is the thermal diffusivity, epsilon is the volumetric expansion ratio, S-L is the laminar burning velocity, c(g) is the model constant, and alpha is the acceleration exponent. The present model expresses model constant c(g) using the properties of gas mixture. In this study, field experiments of gas explosion are conducted for hydrogen/air, methane/air, and propane/air mixtures confined in a 1- or 27-m(3) regular cubic plastic tent. The experimental results demonstrate the nature of self-similarity in the explosions and the experimental acceleration exponent associated with a fractal dimension is evaluated. The model is developed by using the concept of self-similarity and an acoustic theory. The predicted flame speed and the blast pressure are compared with experimental data of larger-scale hydrogen/air, methane/air and propane/air explosions under a wide range of conditions. The model predictions agree reasonably well with the experimental data, validating the proposed model. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.