Duct size chart (CFM by diameter)
Airflow each round galvanized duct carries at common friction rates, calculated with the same formula as the calculator.
| Round duct | CFM @ 0.06 | CFM @ 0.08 | CFM @ 0.10 |
|---|---|---|---|
| 4" | 30 | 35 | 35 |
| 5" | 50 | 60 | 65 |
| 6" | 85 | 95 | 110 |
| 7" | 125 | 145 | 165 |
| 8" | 180 | 205 | 230 |
| 9" | 240 | 280 | 315 |
| 10" | 320 | 370 | 420 |
| 12" | 520 | 605 | 680 |
| 14" | 780 | 905 | 1,020 |
| 16" | 1,110 | 1,290 | 1,450 |
| 18" | 1,515 | 1,760 | 1,980 |
| 20" | 2,000 | 2,325 | 2,615 |
Flexible duct has more friction than smooth metal. If you use flex, stretch it tight and go one size larger than the chart.
How duct sizing works
Most residential systems are designed with the equal-friction method: every duct is sized so that air loses about the same pressure per 100 feet. ACCA Manual D sets the friction rate from the blower's available static pressure and the longest duct run; 0.08 inches of water column per 100 feet is a common result for a typical house.
The calculator finds the diameter that gives that friction rate for your airflow, using the standard friction formula for galvanized steel duct. It then checks the air velocity and, if the air would move too fast for the duct section (which causes noise), increases the size.
Each ton of air conditioning moves about 400 CFM. A 3-ton system therefore needs about 1,200 CFM through the main supply trunk, split among the room branches in proportion to each room's load.
Recommended air velocities
| Duct section | Maximum velocity | Why |
|---|---|---|
| Supply trunk | 900 FPM | Higher speeds become audible near the air handler |
| Branch to a room | 600 FPM | Keeps registers quiet in bedrooms and living spaces |
| Return duct | 700 FPM | Return grilles are close to living areas |
Round vs rectangular duct
Round duct moves the most air for the least metal and friction. Rectangular duct fits between joists and inside soffits. A rectangular duct carries the same air as a round duct of equal equivalent diameter, calculated as 1.30 × (a × b)0.625 ÷ (a + b)0.25. Keep the long side no more than about four times the short side; flatter ducts lose more pressure.
Frequently asked questions
How many CFM does a 6 inch duct carry?
About 95 CFM at a friction rate of 0.08 in. wc per 100 ft. That is enough for a small bedroom or bathroom.
What size duct do I need for 400 CFM?
About a 10-inch round duct at 0.08 friction, or a rectangular equivalent such as 12 × 8 inches.
What size trunk line for a 3-ton system?
A 3-ton system moves about 1,200 CFM. That needs about a 16-inch round trunk, or a rectangular trunk such as 22 × 10 inches, at 0.08 friction and under 900 FPM.
How many CFM per square foot?
A rough rule is 1 CFM per square foot of floor area, but rooms with large windows or west exposure need more. A Manual J room-by-room load gives the real figure.
Does flex duct need to be bigger?
Yes. Flexible duct has higher friction than metal, especially when it sags or bends. Most designers go one size up for flex runs.
These results are planning estimates for smooth galvanized duct. A full ACCA Manual D design accounts for fittings, run lengths and the blower's static pressure.