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

Return air duct size

Return air duct size for 1-5 ton ACs is 14” to 22” for round ducts and 13”x12” to 32”x12” for rectangular ducts.

Use the return air duct size calculator below for accurate dimensions required for your air conditioner or heat pump.

There is also a table of return air duct sizes for round and rectangular ducts for ACs from 1.5 to 5 tons.

Please note this article only discusses the main return air duct for you HVAC system. If you are looking for a chart or calculator to size the branch duct, duct for each room or supply air duct, our comprehensive HVAC duct calculator and charts will be what you need.

Return Air Duct Size Calculator

Click and drag the blue dot to the size of your AC or heat pump.

Choose whether you want the calculation for round or rectangular ducts. It’s simple and quick.

The calculator gives you all common return air duct sizes for rectangular ducts plus the correct size for round return ducts.

Return Air Duct Size Calculator

Return Air Duct Size Table

Find your condensing unit size in the left column. It shows all standard sizes for ACs and heat pumps.

The middle column shows Return Duct Size for round ducts, and the right column shows your options for rectangular Return Duct Size.

Return air duct size chart

AC Unit CapacityReturn Duct Size (round duct diameter)Return Duct Size (rectangle duct 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″

What return air duct size for a 3 ton AC?

18” for a round return air duct, and 16”x15” to 40”x7” for a rectangular return air duct.

What size round return air duct do I need for a 4 ton AC?

19” in diameter, according to the return air duct size chart.

Sizing Rectangular Cold Air Return Ducts

Rectangular ducts can be various sizes for the same AC unit as long as the number of square inches is nearly the same.

For example, the right size 3 ton AC cold air return duct ranges from 16”x15” to 26”x10” to 40”x7”. What these three sizes have in common is that they are all close to 260 square inches if you do the calculation – 240/246/280 square inches respectively.

Note: A 3 ton AC needs a round return duct with an 18” diameter. That size duct is 254 square inches, right in line with the calculation for rectangular ducts. 


What is a return air duct?

In this discussion, it is the main duct connected to the air handler, which might also be a furnace. 
A home may have multiple small return air ducts that feed into the main cold air return duct. The calculator and chart above give sizing for the main cold air return duct that feeds air directly into the air handler.

Where is the return air duct?

Locate your air handler. It will have two major ducts connected to it, the supply duct and the return duct.
Once you determine which way air flows through it, you’ll know which is the return air duct – the duct bringing air back to the air handler to be treated (cooled/heated).

Tip for determining airflow direction: If the air handler has a filter in it, the filter should have an arrow on it showing airflow direction.
If the air conditioner is running, then the return air duct should be warmer than the supply duct. If the air handler is a furnace and is heating, then the supply duct will be warm and the cold air return duct will be cooler.

Why does the return air duct size matter?

Because it must be the right size to handle the CFM airflow of the air handler blower. CFM is cubic feet per minute. ACs need roughly 400 CFM per ton.

Undersized: There will be too much pressure in the ductwork, which will be noisy and might eventually cause it to come apart. Your system will work harder. And your rooms won’t get enough air to be as cool or as warm as you want them to be.

Oversized: There will be too little air pressure in the ductwork to move the air, and it won’t get to the rooms, especially those that are furthest from the air handler. You will have a significant temperature imbalance from room to room in your house.

Does the return air duct size calculator work for heat pumps too?

Yes. A heat pump is an AC in summer. In AC mode, it collects heat indoors and pumps it outside. In Heat mode, it collects heat outdoors and pumps it inside.
So whether you have an AC-only system or a heat pump, you can use the calculator and the return air duct size chart.

I have a manufactured home and can’t find the return air duct.

Many manufactured homes, aka trailer homes, do not have return air ducts. The design of the house doesn’t allow them. Instead, the air flows back to the air handler or furnace without ductwork. That’s why there probably is not a door – or there shouldn’t be a door – leading into the room where the furnace is installed.

What is a return air vent?

It is a vent through which return air is pulled into the ducts that feed into the main return air duct in your home. Most homes have several return air vents but only one main cold air return duct.

Note: Cold air return air duct is something of a misnomer. When heating your home, yes, the return duct carries cold air back to the air handler or furnace to be heated. But in AC mode, the air in the “cold air” duct is actually warmer than air carried through supply ducts. 

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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