This document specifies methods for the measurement of air flow rates on site. It provides a description of the air flow rate measurement methods and how measurements are performed within the margins of stipulated method uncertainties. It gives the necessary measurement conditions (e.g. length of straight duct, uniform velocity profile) to achieve the stipulated measurement uncertainties. The methods for measuring the air flow rate inside ducts do not apply to: ducts that are not circular or rectangular (e.g. oblong ducts); flexible ducts.