Enter the size of the air conditioner (tons) into the calculator to determine the size of the return air duct.

Return Air Duct Size Formula

The following formulas are used to calculate the return air duct size of a rectangle and circular air duct.

Drect = SQRT(AS * 144)

  • Where Drect is the length and width of the return duct size (inches)
  • AS is the size of the air conditioner (tons)

Dcircle = 2 * SQRT (AS*144 / PI)

  • Where Dcircle is the diameter of the circle (inches)
  • A is the air conditioning size

What is a return air duct size?

A return air duct is a square or circular column used to return air into an airconditioner.

The size of the air duct is how large that duct should be based on the size of the air conditioner.

How to calculate return air duct size?

The following example outlines the steps required to calculate a return air duct size.

First, determine if you want a rectangular or circular return duct. For this example, the return duct will be rectangular.

Next, determine the size of the air conditioner. This can be found on the back of the air conditioner. In this case, the size is 3 tons.

Finally, calculate the return duct size using the formula above:

Drect = SQRT(AS * 144)

Drect = SQRT(3 * 144)

Drect = 20.78″

So the the air duct should be roughly 21″ x 21″