Duct Size Chart by CFM: Round and Rectangular Duct Sizes
Part of the project planner: Planning heating, cooling and ventilation.
A 6 inch round duct carries 100 CFM, an 8 inch duct carries 200 CFM and a 10 inch duct carries 400 CFM, at the usual design friction rate of 0.10 inches of water per 100 feet. CFM is cubic feet of air per minute. The duct has to be big enough to move the air without too much noise or pressure loss. The duct size calculator gives the round diameter, the next standard size, the air speed and the other side of a rectangular duct.
Round duct size by CFM
The diameter for three friction rates, the next standard size at 0.10 and its air speed:
| Airflow | 0.08 (quiet) | 0.10 (usual) | Standard size at 0.10 | Air speed | 0.15 (tight) |
|---|---|---|---|---|---|
| 50 CFM | 4.7 in | 4.5 in | 5 in | 367 fpm | 4.1 in |
| 75 CFM | 5.5 in | 5.2 in | 6 in | 382 fpm | 4.8 in |
| 100 CFM | 6.1 in | 5.8 in | 6 in | 509 fpm | 5.4 in |
| 150 CFM | 7.1 in | 6.8 in | 7 in | 561 fpm | 6.3 in |
| 200 CFM | 7.9 in | 7.6 in | 8 in | 573 fpm | 7.0 in |
| 300 CFM | 9.2 in | 8.8 in | 9 in | 679 fpm | 8.1 in |
| 400 CFM | 10.3 in | 9.8 in | 10 in | 733 fpm | 9.1 in |
| 600 CFM | 12.0 in | 11.5 in | 12 in | 764 fpm | 10.6 in |
| 800 CFM | 13.4 in | 12.8 in | 14 in | 748 fpm | 11.8 in |
| 1000 CFM | 14.5 in | 13.9 in | 14 in | 935 fpm | 12.8 in |
| 1200 CFM | 15.6 in | 14.9 in | 16 in | 859 fpm | 13.7 in |
| 1600 CFM | 17.4 in | 16.6 in | 18 in | 905 fpm | 15.3 in |
| 2000 CFM | 18.9 in | 18.1 in | 20 in | 917 fpm | 16.7 in |
Rectangular duct with the same flow
Friction is the same for a rectangle and a round duct with the same equivalent diameter. The other side for a given height, at 0.10:
| Airflow | Height 6 in | Height 8 in | Height 10 in | Height 12 in |
|---|---|---|---|---|
| 100 CFM | 4.7 in | 3.7 in | 3.1 in | 2.7 in |
| 200 CFM | 8.0 in | 6.0 in | 4.9 in | 4.3 in |
| 400 CFM | 14.1 in | 10.1 in | 8.1 in | 6.9 in |
| 800 CFM | 25.7 in | 17.7 in | 13.7 in | 11.4 in |
| 1200 CFM | 37.2 in | 25.1 in | 19.0 in | 15.5 in |
| 1600 CFM | 48.6 in | 32.3 in | 24.2 in | 19.5 in |
Keep the shape fairly square: a rectangle more than four times as wide as it is tall wastes metal and adds friction.
How much air does each system need?
Air conditioners and heat pumps move about 400 CFM per ton of cooling. Main duct sizes for common systems at 0.10:
| System | Airflow | Duct needed | Standard size |
|---|---|---|---|
| 1 ton | 400 CFM | 9.8 in | 10 in |
| 1.5 ton | 600 CFM | 11.5 in | 12 in |
| 2 ton | 800 CFM | 12.8 in | 14 in |
| 2.5 ton | 1,000 CFM | 13.9 in | 14 in |
| 3 ton | 1,200 CFM | 14.9 in | 16 in |
| 4 ton | 1,600 CFM | 16.6 in | 18 in |
| 5 ton | 2,000 CFM | 18.1 in | 20 in |
Share the total between the rooms by the heat each one needs, using the AC size calculator for each room.
What the numbers mean
- Friction rate: the pressure lost per 100 feet of straight duct, in inches of water. 0.08 is quiet and 0.10 is the usual design value. The rate for a whole system depends on the fan and the longest run.
- Air speed: the airflow divided by the area of the duct. Branches near 600 to 700 fpm are quiet.
- Fittings: elbows, tees and dampers add the equivalent of extra duct. A 90 degree elbow is roughly 10 to 15 ft of straight duct.
- Flexible duct has more friction than metal duct. Keep it short and fully stretched, or go up a size.
- Register sizes are chosen after the ducts, from the airflow and the throw you need.
Sealing and insulating
- Seal every joint with mastic or foil tape rated for ducts. Leaky ducts can waste a large share of the heated or cooled air.
- Insulate ducts in an attic or crawlspace, to keep them from sweating and to keep the heat in.
- Return air matters as much as supply: undersized returns make the fan work harder and the room noisy.
This is the equal-friction method for sizing a duct from an airflow. A full system design (ACCA Manual D) also counts fittings, the fan curve and the room loads, and is the standard for new systems. For a bathroom or kitchen fan, use the CFM calculator.
Frequently asked questions
What size duct do I need for 100 CFM?
About 5.8 inches round at the usual friction rate of 0.10, so a 6 inch duct. The air then moves at 509 feet per minute.
What size duct for 400 CFM?
About 9.8 inches round, so a 10 inch duct. That is the flow of a 1 ton air conditioner at 400 CFM per ton.
What size duct for a 3 ton air conditioner?
A 3 ton system moves about 1,200 CFM, which needs a main duct of 14.9 inches, so 16 inches round, or an equivalent rectangle.
How do I convert round duct to rectangular?
Use the equivalent diameter formula: 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Our calculator solves it for the second side when you give the first. A square or near-square duct is more efficient than a flat one.
What air speed is right for a duct?
About 600 to 700 feet per minute in a residential branch and up to about 900 in a trunk. Faster air is noisier and takes more fan power.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.