The return air duct has to carry all the air your system supplies, and at a lower speed than the supply side.
At 400 CFM per ton and the 700 fpm maximum ACCA Manual D sets for a return trunk, a 2 ton unit needs a 16 inch round return duct, a 3 ton unit an 18 inch and a 4 ton unit a 22 inch. The same airflow can also be split across two or more smaller returns.
Undersized return air ducts make the ducts and grilles noisier than usual and put the air conditioner under strain.
Use the return air duct size calculator or the chart below to find the correct return air duct size for your home.
These cover the return air duct only. For supply ducts, see our duct size chart and calculator.
Updated October 2026: the calculator and chart now apply the ACCA Manual D velocity limit for return ducts, which makes several sizes one step larger than before.
Return Air Duct Size Calculator
Enter your unit size in tons, or the airflow in CFM if you know it. If you are not sure of the tonnage, see how to find your air conditioner size.
Duct size calculatorUses ACCA Manual D velocity limitsRecommended size18 inround sheet metal duct for a return trunkSized up to stay under 700 fpm3 tons x 400 CFM per ton = 1,200 CFMDuctA branch is the run to one room.ShapeMaterialRectangular sizes are for sheet metal.Friction rateIn. w.c. per 100 ft. 0.06 gives larger ducts; 0.10 is the Manual D example.Airflow1,200 CFMAir velocity679 fpmFriction per 100 ft0.036 in. w.c.Recommended up to 600 fpmMaximum 700 fpmA 16 in duct meets the friction rate, but the air would move at 859 fpm. That is over the 700 fpm Manual D maximum for a sheet metal return trunk, so the size goes up to 18 in.Same airflow in rectangular sheet metal (width x height)32 x 8 in 675 fpm26 x 10 in 665 fpm18 x 14 in 686 fpm16 x 16 in 675 fpmReturn grille free areaat least 346 sq in (500 fpm)Filter grille free areaat least 576 sq in (300 fpm)Formula: friction loss per 100 ft = f x (1200 / D) x 0.075 x (V / 1097)2, where V is CFM divided by the duct area and f comes from the Altshul-Tsal equation. The answer is the smallest standard size at or under your friction rate, moved up a size if the air would be faster than the Manual D maximum.
Rectangular sizes use the Huebscher equivalent diameter, 1.30 x (a x b)0.625 / (a + b)0.25.
Sources: ACCA Manual D for the friction rate method and the Table 3-1 velocity limits, and the ASHRAE Handbook, Fundamentals, for the friction equations and duct roughness (0.0003 ft for galvanized steel, 0.015 ft for flex), as printed in LBNL-49012. 400 CFM per ton is the usual rule of thumb.
This is a sizing guide for straight duct. A permit or a new system needs a full ACCA Manual D calculation, which adds up the fittings and uses your blower’s own data.
Return Air Duct Size Chart
| AC unit capacity | Airflow (CFM) | Round sheet metal | Round flex | Rectangular sheet metal (WxH) | Or two round returns | Filter grille free area |
|---|---|---|---|---|---|---|
| 1.5 Ton | 600 | 14 inch | 14 inch | 22×6, 16×8, 14×10, 12×12 | 2 x 10 inch | 288 sq in |
| 2 Ton | 800 | 16 inch | 16 inch | 22×8, 18×10, 14×12, 14×14 | 2 x 12 inch | 384 sq in |
| 2.5 Ton | 1,000 | 18 inch | 18 inch | 26×8, 22×10, 18×12, 16×14 | 2 x 12 inch | 480 sq in |
| 3 Ton | 1,200 | 18 inch | 18 inch | 32×8, 26×10, 18×14, 16×16 | 2 x 14 inch | 576 sq in |
| 3.5 Ton | 1,400 | 20 inch | 20 inch | 30×10, 24×12, 22×14, 18×16 | 2 x 14 inch | 672 sq in |
| 4 Ton | 1,600 | 22 inch | 22 inch | 34×10, 28×12, 24×14, 22×16 | 2 x 16 inch | 768 sq in |
| 5 Ton | 2,000 | 24 inch | 24 inch | 36×12, 30×14, 26×16, 24×18 | 2 x 18 inch | 960 sq in |
Sizes are the smallest standard duct that meets both the friction rate and the velocity limit, for a straight run.
The grille figure is free area, the open space between the bars, at Manual D’s 300 fpm limit for a filter grille. The grille maker’s data sheet gives the free area for each size.
Frequently Asked Questions
What return duct size do I need for a 2 Ton unit?
For a 2 ton unit (800 CFM) use a 16 inch round duct, in sheet metal or flex, or one of these rectangular ducts: 22 x 8, 18 x 10, 14 x 12 or 14 x 14 inches.
Two 12 inch returns do the same job.
What is the Duct Size For a 4 Ton Unit?
For a 4 ton unit (1,600 CFM) use a 22 inch round duct, or one of these rectangular ducts: 34 x 10, 28 x 12, 24 x 14 or 22 x 16 inches.
You can also split it: two 16 inch returns carry the same air.
What size return air duct for a 3 ton or 5 ton unit?
A 3 ton unit (1,200 CFM) needs an 18 inch round return duct, or a rectangular duct of 32 x 8, 26 x 10, 18 x 14 or 16 x 16 inches.
A 5 ton unit (2,000 CFM) needs a 24 inch round return, or two 18 inch returns.
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 as the supply duct, and preferably bigger.
The reason is air speed. ACCA Manual D allows up to 900 fpm in a sheet metal supply trunk but only 700 fpm in a return trunk, so the same airflow needs a larger return.
What happens if The return air vent is too small?
If the return air duct is too small, the air has to move faster to get through it. That means excess noise (possibly whistling) and a fan that works harder.
ACCA also warns that wrong-sized ducts can make the fan fail sooner and can cause the equipment’s safety devices to shut it down.
What size return air grille do I need?
Size the grille by its free area, not its outside dimensions. Manual D limits the face velocity to 500 fpm for a plain return grille and 300 fpm for a filter grille.
For each ton (400 CFM) that works out to about 115 square inches of free area for a plain grille, or 192 square inches for a filter grille.
A steel return air filter grille holds a 1 inch filter behind a removable face. Check the maker’s free area figure before you order.
To check what you already have, hold an anemometer against the grille and read the air speed in feet per minute.
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.
