The short answer
At a 0.1 friction rate, 150 CFM takes a 7 inch round supply branch (561.26 feet a minute) or a 9 inch return branch, because return air should move slower. A 1,200 CFM supply trunk takes 18 inches: 16 inches meets the friction rate, but the air would move faster than ACCA's recommended 700 feet a minute.
Round duct size by airflow
Rigid metal, 0.1 in. per 100 ft friction rate, ACCA recommended velocities. Standard sizes: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24 in.
| Airflow | Supply branch | Supply trunk | Return branch | Return trunk |
|---|---|---|---|---|
| 50 CFM | 5 in | 5 in | 5 in | 5 in |
| 75 CFM | 6 in | 6 in | 6 in | 6 in |
| 100 CFM | 6 in | 6 in | 7 in | 6 in |
| 150 CFM | 7 in | 7 in | 9 in | 7 in |
| 200 CFM | 8 in | 8 in | 10 in | 8 in |
| 300 CFM | 10 in | 9 in | 12 in | 10 in |
| 400 CFM | 12 in | 12 in | 14 in | 12 in |
| 600 CFM | 14 in | 14 in | 18 in | 14 in |
| 800 CFM | 16 in | 16 in | 20 in | 16 in |
| 1,000 CFM | 18 in | 18 in | 22 in | 18 in |
| 1,200 CFM | 20 in | 18 in | 24 in | 20 in |
| 1,600 CFM | 24 in | 22 in | Over 24 | 24 in |
How it works
The formula
A duct is big enough when the air loses no more pressure to friction than the blower has to spare, and moves slowly enough to stay quiet. We find the smallest standard round size that does both.
speed (feet a minute) = CFM ÷ duct area (sq ft)
rectangular equivalent = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25
Assumptions
- Friction is the standard fit to the ASHRAE friction chart for round galvanized duct and standard air. Rectangular equivalents use the Huebscher equation from the ASHRAE Handbook.
- Velocities, feet a minute, from ACCA Manual D Table 3-1 as reproduced in ACCA's duct design checklist: supply trunk 700 recommended, 900 max (flex 600, 700); supply branch 600, 900; return trunk 600, 700; return branch 400, 700. We size to the recommended speed.
- The friction rate is per 100 ft of total effective length, which counts fittings as extra feet; Manual D works it out from the blower's available static pressure.
Worked example: 1,200 CFM supply trunk
- Friction: 0.109136 × 1,200^1.9 ÷ d^5.02 ≤ 0.1 → d ≥ 14.9 in → 16 in
- Speed in 16 in: 859.44 feet a minute, over the 700 recommended
- 18 in round: 679.06 feet a minute, or 60 × 6, 40 × 8, 30 × 10, 24 × 12 in rectangular
Too big or too small
- Too small: noise and weak airflow. Fast air whistles at the registers and the blower can't push enough air, so rooms at the end of the run stay hot or cold.
- Too big: slow air that doesn't reach. Oversized branches drop the speed so far the air doesn't mix the room; supply grilles need some throw.
- Returns matter as much as supplies. An undersized return starves the blower; ACCA recommends slower return air, so returns run bigger.
Questions people ask
What size duct do I need for 150 CFM?
A 7 inch round supply branch at a 0.1 friction rate: about 561.26 feet a minute, under ACCA's 600 recommended. As a return branch it takes 9 inches, because ACCA recommends slower return air (400 feet a minute) to keep it quiet.
How big should the main trunk be?
Size it for all the air it carries. A trunk for a 1,200 CFM system needs 18 inch round: 16 inches would meet the friction rate but move air at 859.44 feet a minute, over the 700 ACCA recommends for a supply trunk.
What friction rate should I use?
It comes from the blower: the static pressure left after the coil, filter and grilles, spread over the longest duct run. ACCA's Manual D worksheet works an example out to 0.1 in. per 100 ft, a common value; a longer system or a restrictive filter means a lower rate and bigger ducts.
Can I use flex duct?
Yes, for branches, but it has more friction than smooth metal, and much more when it sags or is compressed. These sizes are for metal: with flex, go at least one size up, pull it tight, support it every 4 to 5 ft and keep bends gentle. ACCA also caps flex air speed lower, at 700 feet a minute.
Round or rectangular?
Round duct moves the most air for the metal and has the least friction. Rectangular fits in joist bays and low spaces; the equivalent sizes shown carry the same air with the same friction.