How to Estimate How Much Ductboard You Need for HVAC

Measure the inside of your existing ducts. Duct measurements are based on interior dimensions.

Determine the necessary length of your duct system by measuring existing ductwork or by consulting any construction diagrams you may have. Ductwork may be visible in the basement or attic. Measure the distances between the system and room registers where the duct system serves multiple floors.

Determine which size ductboard to purchase based on the size of the ductwork you are fabricating and the need for insulation. Ductboard is 4 feet wide but comes in either 96- or 120-inch lengths and in thicknesses ranging from 1 to 2 inches. When creating a duct that has an interior measurement of 14 by 6 inches from 1-inch-thick board, for instance, you need to add 1 3/4 inches to each measurement to account for the joint and the depth of the board, which is cut at a diagonal. Based on these figures you would need 47 inches of board for one segment of duct, meaning you could make two modules from a 96-inch piece of duct board. Ducts are always fabricated along the length of the board to keep segments in manageable, standardized 4-foot pieces.

Divide your length measurement from Step 2 by 47.125 for 1-inch board, by 46.625 for 1 1/2-inch board and by 46.125 for 2-inch board. This will tell you how many of the modules you will need. According to the North American Insulation Manufacturer's Association, you don't calculate based on the full 48-inch length of the modules created from the 48-inch-wide boards because you have to subtract the length of the joint needed between each segment.

Divide the number by the number of modules you can create from each board, determined in Step 3, to arrive at how many boards you need to purchase to complete your ductwork.