This International Standard deals with the measurement of the heat value of natural gases and synthetic natural gases using non-separating methods, ie methods that do not require a determination of the gas composition or a calculation from it. The standard describes the principles of operation of a selection of measuring instruments useful for this purpose and provides instructions for the selection, evaluation, performance assessment, installation and operation of these. Heat values can be given based on mass, substance or volume, the latter being the most common form. The working range for the volume-related calorific value of natural gas is usually between 30 MJ/m3 and 45 MJ/m3 at standard reference conditions (see ISO 13443). The associated range for the Wobbe index is usually between 40 MJ/m3 and 60 MJ/m3. This International Standard neither confirms nor disputes any commercial manufacturer's claims regarding the performance of a measuring instrument. The central claim is that the suitability for a particular application (defined in terms of a set of specific operational requirements) can only be assessed through a carefully designed program of experimental investigations. Instructions regarding the appropriate content are provided for these exams.