A hybrid power train, consisting of a Polymer Electrolyte Fuel Cell (PEFC) system and batteries, which feeds an electric motor for city bus propulsion, can be dimensioned ad hoc respect to the performed route, avoiding his oversizing in the greater energy rationalization optic. In this article a calculation tool is developed and implemented in Matlab (R) and Simulink (R) environments in such a way that it can be used both for verifying the electric motor proper operation in the route and for dimensioning the hybrid system (PEFC system and batteries), after that city bus characteristics and route have been defined. It considers the electric energy recoverable by city bus both downhill and in the deceleration phase (regenerative braking), can simulate various traffic conditions and can be used to obtain a good estimate of both the hydrogen amount on city bus board and the batteries state of charge along the route. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.