This International Standard provides a method for the determination of gas velocity and volume flow rate within an emissions duct. It describes a method to determine the velocity profile of the gas flow at a point in the duct, and a method to determine the total volume flow rate at a measurement plane in the duct based on a grid of point velocity measurements made across the measurement plane. In addition, alternative methods are described in the annex of the standard for the determination of volume flow rate based on the measurement of tracer dilution, tracer transit time, and by calculation from energy consumption. This International Standard provides quality control checks for the verification of the conditions for accurate measurements. The volume flow rate may be reported at duct conditions or may be expressed at standard conditions (273,15 K and 101,325 kPa) on either the wet or dry basis. The described method, which has been validated in the framework of field trials at a municipal waste incinerator in Denmark and a coal-fired power plant in Germany, can typically attain measurements with an uncertainty of 5 % at flow velocities of 20 m×s-1. The standardization work has taken place in the framework of mandate of the EU and EFTA, which have also financially supported the performance of validation tests. The International Standard has been prepared by Working Group CEN/TC 264/WG 23 "Manual and automatic measurement of velocity and volumetric flow in ducts" in Technical Committee CEN/TC 264 "Air quality". The committee responsible on a national level is Subcommittee NA 134-04-01-16 UA "Volumenstrombestimmung" ("Determination of volumetric flow") of Technical Division IV "Umweltmesstechnik" ("Environmental measurement techniques") of the Commission on Air Pollution Prevention of VDI and DIN - Standards Committee KRdL.