Hydrogen is a promising alternative for current energy carriers. Compressed gas cylinders are the storage systems closest to the commercialization of hydrogen in vehicles. The safety factors in current standards are seen as restrictive for further growth and competitiveness of hydrogen infrastructure. A probabilistic approach can be employed in order to give a rational background to the safety factors. However, an acceptable probability of failure needs to be estimated before calculating the safety factors. The discussion of determining the acceptable probability must include the mass of hydrogen since this determines the consequences of an accident. It is concluded that an annual probability of failure of 10(-7) would be appropriate for small pressure vessels containing a few kilograms of hydrogen. Larger pressure vessels of a few hundred kilograms or more should be designed for an annual probability 10(-8). Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.