Over-the-Range Microwave - Ducting Information

Duct Installation Guidelines:

All over-the-range microwaves can be vented to the outside using an equivalent total of 140 feet of rigid 3 1/4" x 10" rectangular or rigid 6" round ducting.


If using rectangular ducting, please note:

  • Microwaves are shipped from the factory to vent out the top (vertically) but can also be vented out the rear (horizontally).

  • The duct must be the sizes listed above for proper draw of air. Both smaller and larger vents than recommended decrease venting performance.

  • Microwaves cannot be ducted into a common duct work.

  • DO NOT vent into an attic or crawl space.


If using round ducting, also note the following:

  • A rectangular to round transition adapter must be used.

  • Round ducts should be a minimum of six inches (can use seven) in diameter.

  • The use of corrugated cardboard or plastic duct is not recommended.


The equivalent total feet is the sum of all elbows, transitions, wall caps, adapters, etcetera, plus the total length of straight duct used. The following list provides a few examples of the equivalent feet of certain ductwork. Deduct the footage in the right column from 140 feet for each item used.


Transition Adapter5 feet
Wall Cap40 feet
Roof Cap24 feet
90 Degree Rectangular Elbow25 feet
45 Degree Rectangular Elbow5 feet
90 Degree Round Elbow10 feet
45 Degree Round Elbow5 feet


If the equivalent duct lengths exceed 140 feet, a transition adaptor should be used to convert the ductwork to seven or eight inch round duct. The larger size duct will compensate for the longer runs to maintain satisfactory airflow performance. Up to 150 equivalent feet of seven inch round or up to 250 equivalent feet of eight inch round duct can be used.


Duct Purchasing Recommendations:

Materials needed to vent the microwave can be purchased at plumbing supply houses, hardware stores, home improvement stores, heating and air conditioning companies, etcetera. Transition adapters, if needed for installation, can be purchased from building supply stores.