The cone calorimeter is one of the major fire tests. The measurement method is based on the evaluation of mass flow and oxygen concentration of fire effluents to calculate heat release rate. Different studies highlighted that the governing parameter for uncertainty at important values of heat release rate was the characteristic constant of an orifice plate used to measure mass flow (C-factor). This parameter is usually determined each testing day by oxygen consumption calorimetry with a reference methane burner. This publication presents a calibration method for volumetric flow rate without using orifice plate and then an extension to C-factor determination without methane burner and calorimetry. The uncertainty calculation applied to a real example highlights the fact that the method is suitable to respect tolerance of standardized test conditions for both parameters. Copyright © 2013 John Wiley & Sons, Ltd.