What Size Duct for a 3-Ton AC Unit?

Duct size isn't driven by tonnage — it's driven by airflow. The tonnage just tells you the airflow target. Residential cooling equipment is designed around 400 CFM per ton of nominal capacity, so a 3-ton condenser paired with a matching air handler or furnace needs roughly 1,200 CFM through the duct system. Manufacturers publish the acceptable band on the blower data plate — usually 350 to 450 CFM per ton — so a 3-ton system realistically needs the ducts to move somewhere between 1,050 and 1,350 CFM.

Why the floor matters: fall much below 350 CFM per ton and suction pressure drops, coil temperature falls, and you're headed toward a frozen evaporator. Hamilton Home Products' installer guidance puts it plainly — a minimum of 400 CFM per ton to keep the coil from freezing up. In dry climates some techs run 425–450 CFM per ton for more sensible capacity; in humid climates 350 CFM per ton pulls more moisture. Either way, size the ducts for the airflow the equipment actually needs, then confirm against the blower table.

Two limits control the answer: friction and velocity. ACCA Manual D (Table N3-1) caps supply trunks at 900 FPM and returns at 700 FPM in residential work — beyond that you get air noise at the registers and burn static pressure the blower doesn't have.

Supply trunk. At 1,200 CFM, a 16 in. round metal trunk runs about 860 FPM (16 in. round = 1.40 sq ft of area) and lands around 0.07–0.08 in. wc per 100 ft of friction — comfortably inside both limits. If your friction-rate calc comes out low (long runs, restrictive filter, several elbows), step up to 18 in. round. Common rectangular equivalents: 18x12 (≈16.0 in. equivalent diameter) or 20x10 (≈15.2 in.). The shallow-joist favorite 22x8 works out to only about a 14.1 in. equivalent — it shows up on contractor charts at 1,200 CFM, but it's marginal; treat it as the floor, not the target.

Return. Return airflow must equal or exceed supply airflow, and the velocity cap is tighter (700 FPM). At 1,200 CFM a 16 in. round return runs ~860 FPM — too fast. Go 18 in. round (~680 FPM) or 16x20 rectangular (~540 FPM). LearnMetrics' return-sizing tables list 18 in. round, 20x12, 22x10, or 30x8 for 3-ton systems — all in the same range. An undersized return is the most common airflow killer on 3-ton changeouts: the old 14 in. return that "worked" with the previous PSC blower will choke a new high-efficiency system.

Branches. A 3-ton house typically runs ten to fourteen supply branches. At a 0.1 in. wc/100 ft friction rate, a 6 in. round branch carries roughly 100–115 CFM and a 7 in. carries roughly 160–170 CFM — so plan branch count from the room-by-room loads, not by copying the old duct layout.

Capacity of round galvanized duct at the two friction rates most residential designs land on. Computed with the ASHRAE Fundamentals duct-friction method (Darcy-Weisbach with the Colebrook equation, 0.0003 ft roughness, 0.075 lb/ft3 air) and cross-checked against the PickHVAC and Hamilton round-duct charts, which agree within a few percent. Read it two ways: pick a CFM and read across for the size, or pick a size and see what it can carry.

Round duct (in.)	CFM at 0.08 in. wc/100 ft	CFM at 0.10 in. wc/100 ft	Velocity at 0.10 (FPM)
4	33	37	425
5	60	68	500
6	98	111	565
7	148	168	630
8	212	240	685
9	291	329	745
10	386	436	800
12	628	709	905
14	947	1,069	1,000
16	1,351	1,524	1,090
18	1,847	2,083	1,180
20	2,443	2,755	1,265
22	3,145	3,545	1,345
24	3,960	4,463	1,420

For 1,200 CFM the table lands you at 16 in. round (1,524 CFM capacity at 0.10, 1,351 at 0.08). Note the velocity column: at 0.10 in. wc the 16 in. duct is already running over 1,000 FPM at full capacity, which is why a 3-ton trunk is sized on the friction rate and then checked against the 900 FPM Manual D limit, not the other way round. At 1,200 CFM the 16 in. trunk runs about 860 FPM. Flex duct carries noticeably less than metal at the same diameter because of the corrugated liner and sag, so size flex one diameter up from this table, and keep it pulled tight.

Same method, applied across the residential range at 400 CFM per ton, 0.08-0.10 in. wc friction, 900 FPM supply / 700 FPM return limits. Use it to sanity-check an existing system before a changeout: a 3-ton condenser bolted onto ducts built for a 2-ton furnace is one of the most common causes of high static, noise and a short-lived compressor.

Tonnage	Airflow (CFM)	Supply trunk (round)	Supply trunk (rectangular)	Return (round)	Return (rectangular)
1.5	600	12 in.	12 x 10	14 in.	14 x 10
2	800	14 in.	14 x 10 or 18 x 8	16 in.	16 x 12
2.5	1,000	14-16 in.	16 x 10 or 20 x 8	16-18 in.	20 x 10
3	1,200	16 in.	18 x 12 or 20 x 10	18 in.	16 x 20
3.5	1,400	16-18 in.	20 x 12	18-20 in.	20 x 14
4	1,600	18 in.	22 x 12 or 20 x 14	20 in.	24 x 14
5	2,000	20 in.	24 x 14	22 in.	24 x 18 or two 16 in. rounds

Where two sizes are listed, the smaller works on a short, straight system with a strong blower and the larger is the safer pick for long runs, restrictive filters or several elbows.

There is no fixed number, because registers are sized to rooms, not to the condenser. In practice a 3-ton system in a 1,500-2,000 sq ft home ends up with 8 to 12 supply registers and 1 to 2 large returns (or a return in every closed room).

The math that gets you there:

• A 6 in. round branch carries about 100-115 CFM at 0.10 in. wc. A 7 in. carries about 150-170 CFM. An 8 in. carries about 220-240 CFM.
• A typical bedroom needs 75-125 CFM, a living room or open kitchen 200-350 CFM, a bathroom 50-75 CFM.
• Add up the room CFMs from a load calculation (Manual J), give each room a branch sized to its number, and the register count falls out. 1,200 CFM divided across 10 rooms is about 120 CFM each, which is why 6 in. and 7 in. branches dominate residential work.

Two rules from the field: every branch needs a register big enough that face velocity stays under about 500-700 FPM (a 4 x 10 floor register handles roughly 100 CFM; a 4 x 12 about 120 CFM), and return grille area should total at least the supply, with total return grille face area around 1 sq in. per 2 CFM so the return side stays quiet. Undersized returns are the number one cause of a 3-ton system that sounds like a jet and never reaches its rated airflow.

You can't convert round to rectangular by matching cross-sectional area. A rectangle has more wall surface per square inch of area than a circle, so it generates more friction at the same airflow. The correct conversion is the Huebscher equivalent-diameter equation from the ASHRAE Handbook — Fundamentals (Ch. 21, Duct Design):

De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25

where a and b are the inside dimensions in inches and De is the round diameter with the same friction loss per foot.

The trap, with numbers: a 20x10 rectangular trunk has 200 sq in. of area — essentially identical to a 16 in. round duct's 201 sq in. But its equivalent diameter is only 15.2 in., so it carries less air at the same friction rate. Match by equivalent diameter, not area, or every rectangular trunk you build comes up short.

Equivalents around the 3-ton range (Huebscher, rounded), with velocity at 1,200 CFM:

Rectangular (in.)	Equivalent round (in.)	Velocity at 1,200 CFM	Verdict for a 3-ton trunk
18 x 12	16.0	800 FPM	Good
20 x 12	16.8	720 FPM	Good, quieter
24 x 10	16.5	720 FPM	Good
30 x 8	16.1	720 FPM	Good, shallow joist bays
20 x 10	15.2	865 FPM	OK, at the limit
22 x 10	15.9	785 FPM	Good
24 x 8	14.6	900 FPM	Marginal
22 x 8	14.1	980 FPM	Too small as the sole trunk

Also keep the aspect ratio sane. Past about 4:1, friction and fabrication cost climb fast, which is why SMACNA's duct-construction standards and most shops favor proportions closer to 2:1.

Every chart in this guide assumes a friction rate — usually 0.1 in. wc per 100 ft. That number is only right if your blower can actually deliver it. The Manual D method (walked through well by HVAC School) computes the real one:

Friction rate = (Available static pressure × 100) ÷ Total effective length

Available static pressure (ASP) is the blower's rated external static pressure minus everything that eats it: wet coil, filter, supply registers, return grilles, balancing dampers. Total effective length (TEL) is the physical duct run plus the equivalent length of every fitting — an elbow can add 15–30 ft of equivalent length by itself.

Example with real-shaped numbers: a blower rated at 0.50 in. wc ESP, minus 0.21 in. wc for a wet coil (when it isn't already included), 0.10 in. wc for a 1 in. filter, and 0.06 in. wc for registers and grilles leaves 0.13 in. wc of ASP. On a 285 ft TEL, the friction rate is (0.13 × 100) ÷ 285 = 0.046 in. wc/100 ft — less than half the 0.1 the rule-of-thumb chart assumed. At 0.046, that 1,200 CFM trunk needs 18 in. round, not 16 in. (PickHVAC's 0.06 column already shows 16 in. maxing out right at 1,200 CFM.)

That's the whole argument: the rule of thumb sizes the duct for a friction rate the system may not have. Short, slick duct systems with low-restriction filters can beat the chart; long runs with 1 in. pleated filters and a dozen fittings will fall short of it. Run the numbers, then pick the size.