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.

The table below is for round sheet-metal duct at a friction rate of 0.1 in. wc per 100 ft, read off the ASHRAE-style friction chart used by contractor references like PickHVAC and the Hamilton duct-sizing chart. Values are approximate — different charts round differently, and flex duct carries meaningfully less at the same diameter (a 6 in. flex run is closer to 80–90 CFM even when pulled tight). Confirm against your ductulator or a friction-rate calculator before cutting metal.

Round duct diameter	Approx. CFM @ 0.1 in. wc/100 ft
6 in.	~100–115 CFM
7 in.	~160–170 CFM
8 in.	~220–240 CFM
9 in.	~300–320 CFM
10 in.	~400–420 CFM
12 in.	~650–680 CFM
14 in.	~1,000–1,050 CFM
16 in.	~1,400–1,450 CFM
18 in.	~1,900–2,000 CFM
20 in.	~2,500–2,600 CFM

Read it for a 3-ton system: 1,200 CFM falls between 14 in. (short) and 16 in. (comfortable) — which is why 16 in. round is the standard call, and why a 14 in. trunk only pencils out if you split the airflow into two trunks early.

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.

Useful equivalents around the 3-ton range (Huebscher, rounded):

Rectangular size	Equivalent round diameter
20x8	≈13.5 in.
22x8	≈14.1 in.
24x8	≈14.6 in.
20x10	≈15.2 in.
18x12	≈16.0 in.
16x20	≈19.5 in.

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.