How to Cut a Hole in Metal Duct Work


The main line of ductwork in a duct system, referred to as the trunk line, carries conditioned air throughout your home. To release the conditioned air from the trunk line, you need to cut holes in the side of the duct to connect branch lines and flexible duct (flex). Cutting holes in metal ductwork requires a variety of tools designed to cut either straight lines or round holes, and during a normal duct installation, you generally need to cut both types of holes in the metal ductwork.

Things You'll Need

  • Tape measure
  • Permanent marker
  • Combination square
  • 1/2-inch drill bit
  • 1/2-inch drill motor
  • Safety glasses
  • Power shear
  • Regular screwdriver
  • Hole cutter

Cutting a Square Hole

  • Measure the width and height of the tap or accessory you need to install in the metal duct.

  • Mark the height and width dimensions from step 1 on the side of the metal ductwork, using a permanent marker.

  • Hold the base of a combination square against one edge of the metal ductwork, and align one edge of the square's blade with one of the width marks you placed on the side of the duct. Drag the permanent marker along the aligned blade edge to draw a line on the side of the ductwork. Slide the combination square to the other width mark and repeat the procedure.

  • Set the combination square to align the end of the square's blade with one of the height marks. Place the permanent marker on the end of the blade, and drag the combination along the edge of the duct to mark your first height line. Readjust the combination square to reach the second height mark and repeat the process. Make sure that the height and width marks intersect, as these will be the cut lines for your hole.

  • Secure the 1/2-inch drill bit into the 1/2-inch drill motor, and put on your safety glasses.

  • Drill a 1/2-inch hole at each corner of the marked hole.

  • Insert the power shear head into the 1/2-inch drill motor. Slide the lower cutting blade of the power shear into a 1/2-inch hole you drilled in the metal duct and cut along the permanent marker line. When you reach a corner, turn the handle of the drill motor 90 degrees and continue cutting along the permanent marker line until you have completed cutting the square hole in the metal duct.

Cutting a Round Hole

  • Measure the diameter of the round tap or collar you need to install in the metal duct.

  • Mark the center of the hole on the side of the metal duct with the permanent marker. Make sure the center of the hole allows you enough room to cut the hole without hitting either corner of the duct.

  • Turn the pivot pin set of the hole cutter with the regular screwdriver to loosen the pilot pin. Slide the pilot pin to the diameter you measured in step 1 and lock the pin in place with the regular screwdriver.

  • Put on your safety glasses and drill a hole on the center you marked earlier, using the hole cutter's drill bit.

  • Slide the pivot pin into the drilled center hole and drill another hole where the drill bit meets the surface of the metal duct.

  • Place your hand on the top handle of the hole cutter, and pull the hole cutter clockwise to cut the round hole in the side of the metal duct. Do not exert excessive pressure on the hole cutter as this could break the hole cutter's drill bit.

Tips & Warnings

  • Metal duct is sharp and can inflict serious cuts if handled improperly; wearing leather work gloves will reduce the amount of cuts you receive while cutting a hole in the ductwork.

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  • Photo Credit ventilation ducts image by Ragne Kabanova from
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