The Dutch Association of Manufacturers of Air Ducts (Luka) considers it essential to define uniform principles when determining the surface area of rectangular air ducts, circular air ducts and fittings.
To this end, formulas have been devised for the most common components, with which the quantity is calculated.

For rectangular ducts and circular ducts larger than 315 mm, so called 'large round' and for fittings the unit of quantity is m2 duct area.
For circular air ducts with a diameter of up to 315 mm, so-called 'small round', this is the length of the duct.

The quantity obtained from calculations may be used for offsetting.

The bases for the formulas are based on the calculation methods used by Luka members and thus on practice.

For rectangular air ducts and 'large round' applies:

• For the purpose of calculating the duct area of any given duct piece, thelargest circumference of the duct opening and the length of the ductpiece, both expressed in metres (m1), shall be determinative. Thelength of the various duct pieces is defined as shown on theaccompanying drawings;
• No deduction will be made for any cuts made in the duct surface for anypurpose;
• Baffles, covers, etc., attached in or to the duct piece shall be addedto the quantity in proportion to their functional area;
• For the calculation of the quantity of internal insulation of straight ducts,expressed in m2, the duct area applies.

For 'small round' applies:

• The length of the duct is determined by measuring over the centre line withmeasurement over valves and fittings. For 90° bends and branches thelength is defined as shown in the accompanying drawing.

The same principles are used to determine the surface area of the duct for the airtightness test, with the understanding that in the case of 'small round' the calculated length is multiplied by the circumference. The same principles are used to calculate the quantity of external insulation, expressed in m2 , with the understanding that the thickness of the insulation is taken into account in determining the circumference.