The Correct Return Air Duct Size for 1-5 Ton Ac Unit (Calculator)

Correctly calculating the return duct size for your AC unit is very important.

Undersized return air ducts result in the ducts and grills being more noisy than usual and the air conditioner components becoming stressed, thus reducing the lifetime of the unit.

Use our handy return air duct size calculator or charts below to help you determine the correct return air vent size for your home.

These charts and calculators are intended to calculate the return air duct only. You can click here to find our standard duct size calculator.

HVAC RETURN AIR DUCT

Return Air Duct Size Calculator

To use this calculator, choose your condensing unit size and you will be given the relevant details.

For a 1 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
12 " Diameter

Rectangular Duct Options: 

  • 11"x10"
  • 12"x9"
  • 14"x8"
  • 17"x7"
  • 20"x6"
  • 28"x5"

Grill Space: 200 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 1.5 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
12 " Diameter

Rectangular Duct Options: 

  • 13"x12"

  • 14"x11"

  • 15"x10"

  • 17"x9"

  • 19"x8"

  • 23"x7"

  • 28"x6

Grill Space: 300 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 2 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
14 " Diameter

Rectangular Duct Options: 

  • 14"x13"
  • 15"x12"
  • 17"x11"
  • 18"x10"
  • 21"x9"
  • 24"x8"
  • 28"x7"
  • 36"x6"

Grill Space: 400 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 2.5 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
16 " Diameter

Rectangular Duct Options: 

  • 15"x14"
  • 17"x12"
  • 19"x11"
  • 21"x10"
  • 25"x9"
  • 29"x8"
  • 37"x7"
  • 41"x6"

Grill Space: 500 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 3 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
18 " Diameter

Rectangular Duct Options: 

  • 16"x15"
  • 17"x14"
  • 20"x12"
  • 23"x11"
  • 26"x10"
  • 29"x9"
  • 34"x8"
  • 40"x7"

Grill Space: 600 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 3.5 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
19 " Diameter

Rectangular Duct Options: 

  • 16"x17"
  • 20"x14"
  • 23"x12"
  • 26"x11"
  • 29"x10"
  • 32"x9"
  • 38"x8"
  • 45"x7"

Grill Space: 700 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 4 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
19 " Diameter

Rectangular Duct Options: 

  • 17"x17"
  • 19"x16"
  • 22"x14"
  • 25"x12"
  • 28"x11"
  • 31"x10"
  • 36"x9"
  • 43"x8"

Grill Space: 800 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 4.5 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
20 " Diameter

Rectangular Duct Options: 

  • 18"x18"
  • 20"x16"
  • 22"x15"
  • 24"x14"
  • 28"x12"
  • 35"x10"
  • 40"x9"
  • 48"x8

Grill Space: 900 square inches

Note: These sizes can be split across multiple return air ducts

 

For a 5 Ton Unit you will require the following return air duct as a minimum :

Round Duct Size:
22 " Diameter

Rectangular Duct Options: 

  • 19"x19"
  • 20"x18"
  • 23"x16"
  • 25"x15"
  • 27"x14"
  • 32"x12"
  • 41"x10"
  • 47"x9"

Grill Space: 1000 square inches

Note: These sizes can be split across multiple return air ducts

 

Return Air Duct Size Chart

AC Unit CapacityRound Duct SizeRectangle Duct Size WxH
1.5 Ton14”13″x12″, 14″x11″, 15″x10″, 17″x9″, 19″x8″, 23″x7″, 28″x6″
2 Ton14”14″x13″, 15″x12″, 17″x11″, 18″x10″, 21″x9″, 24″x8″, 28″x7″, 36″x6″
2.5 Ton16”15″x14″, 17″x12″, 19″x11″, 21″x10″, 25″x9″, 29″x8″, 37″x7″, 41″x6″
3 Ton18”16″x15″, 17″x14″, 20″x12″, 23″x11″, 26″x10″, 29″x9″, 34″x8″, 40″x7″
3.5 Ton19”16″x17″, 20″x14″, 23″x12″, 26″x11″, 29″x10″, 32″x9″, 38″x8″, 45″x7″
4 Ton19”17″x17″, 19″x16″, 22″x14″, 25″x12″, 28″x11″, 31″x10″, 36″x9″, 43″x8″
5 Ton22”19″x19″, 20″x18″, 23″x16″, 25″x15″, 27″x14″, 32″x12″, 41″x10″, 47″x9″

Frequently Asked Questions

What return duct size do I need for a 2 Ton unit?

For a 2 ton unit you can use either a 19″ flex duct or one of the following rectangular ducts: 14″x13″, 15″x12″, 17″x11″, 18″x10″, 21″x9″, 24″x8″, 28″x7″, 36″x6″.

What is the Duct Size For a 4 Ton Unit?

For a 4 ton unit you can use either a 14″ flex duct or one of the following rectangular ducts: 17″x17″, 19″x16″, 22″x14″, 25″x12″, 28″x11″, 31″x10″, 36″x9″, 43″x8″.

Should my return duct size be the same size as the supply duct?

A general rule to follow is that the return air duct should always be AT LEAST as big (and preferably bigger) as the supply duct.

What happens if The return air vent is too small?

If the return air duct is too small then it will result in excess noise (possibly whistles), reduced efficiency and a reduced lifespan of your AC system.

Does My Return Air Vent Need a Filter?

Whether or not your return air vents need filters depends on several factors. If your HVAC system has an existing filter, you don’t need a return vent filter, but you can add them if you like. 

You can learn more about this in our return air vent filter guide.

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Author
Aaron Green
Aaron is the founder of and Essential Home and Garden. He likes to spend his spare time with his family, and doing DIY projects in the home and garden.

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