This International Standard specifies methods for the calculation of gross calorific value, net calorific value, density, relative density, ross Wobbe index and net Wobbe index of natural gases, natural gas substitutes and other combustible gaseous fuels, when the composition of the gas by mole fraction is known. The methods specified provide the means of calculating the properties of the gas mixture at commonly used reference conditions. For the purpose of this International Standard the input mole fractions shall sum to unity exactly. Guidance on the achievement of this requirement is available in ISO 6974-1 and ISO 6974-2. All components with mole fractions greater than 0,000 05 shall be accounted for. If the composition of the gas is known by volume fractions, these shall first be converted to mole fractions in accordance with ISO 14912 (subclause 5.1.2). Note, however, that these derived mole fractions will have uncertainties greater than those of the original volume fractions. The methods of calculation require values for various physical properties of the pure components; these values, together with associated uncertainties, are provided in tables and their sources are identified. Methods are given for estimating the uncertainties of calculated properties. The methods of calculation of the values of properties on either a molar, mass or volumetric basis are applicable to any natural gas, natural gas substitute or other combustible fuel that is normally gaseous, except that for properties on the volumetric basis the method is restricted to mixtures for which the compression factor at reference conditions is greater than 0,9. Example calculations are given in annex D for the recommended methods of calculation.