This standard establishes a method for measuring the surface to quantify the unit of measurement to be used in air duct sheet metal, also including parts. As the total surface is to be understood through the outer surface of the finished surface more specific cross joints. It is applicable to circular ducts and their components, used in ventilation and air conditioning. It is intended to fill the gap in the existing standards taking into account the current manufacturing methods, collecting the measurement criteria used in the Spanish market in recent decades. Not subject to this standard definition of the inner dimensions of pipes, such treaties in the UNE-EN 1505. Not is the subject of this standard the determination of leakage air flow per unit area of the ducts (sealing factor), being necessary to use the UNE-EN 14239.