The construction cost of Ductwork is typically calculated based on the total weight of the ductwork multipled by the cost per lbs. RS MEANS provided a way of calcuation based on the size, gauge, and cost per pound of the ductwork. The gauge of standard ductwork is calculated based on the dimension of the ductwork. Once the gauge is defined, the MEANS table gives the weight of the ductwork in lbs per linear feet based on the sum of two sides of the ductwork. See example below.

For a 40"x18" ductwork, the sum of two side is 58", the long side is 40", therefore use the 54" column with 22 gauge. The weight in lb per foot of length of duct is 14.5 lbs. If the ductwork is 10 ft long, the total weight is 14.5 x 10 = 145 lbs.
 D1 & D2 - Ductwork Cost Estimate.xlsx
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CalcSheet D1
The above tedious manual work could be easily performed by using CalcSheet D1 - Ductwork Weight and Cost Estimate.xlsx.  By entering the dimension & length of the duct, it automatically output its gauge and weight.  There is even an option to override the gauge.
CalcSheet D2
CalcSheet D2 - Ductwork Surface Area, Weight and Cost Estimate.xlsx provide another way to calculation based on weight per ductwork per surface area.  See Sample CalcSheet D2.
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Topic: Ductwork Cost Estimate
HVAC / ENGINEERING 
ENERGY CONSERVATION
FINANCE
BUSINESS
Click to Download D1 Ductwork Weight and Cost Estimate.PDF