A duct has to carry the air your system moves without too much friction, which wastes fan energy, or too much speed, which makes noise. Size it from two numbers: how much air it carries, in cubic feet per minute (CFM), and how much pressure you can afford to lose per 100 feet of duct.
At a common design friction rate of 0.08 inches of water per 100 feet, 400 CFM (roughly one ton of cooling) needs a round duct about 10.1 inches across, and 1,000 CFM needs about 14.3 inches. Round up to the next standard size, then check the air speed.
Duct Sizing Calculator
Round and rectangular duct size from airflow and a design friction rate.
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How the calculator works
- Airflow. Enter CFM, or tons of cooling at a typical design airflow of about 400 CFM per ton.
- Friction rate. The pressure loss you design for, in inches of water per 100 feet; 0.08 is a common starting point, and the right value comes from the system's available fan pressure.
- Duct physics. For smooth galvanized steel duct and standard air, the calculator finds the diameter that produces that friction rate, using the Darcy-Weisbach equation with the Swamee-Jain friction factor. It then rounds up to the next standard size and reports the actual speed and friction.
CFM to round duct size chart
At a friction rate of 0.08 inches of water per 100 feet:
| Airflow | Calculated diameter | Next standard size | Air speed at that size |
|---|---|---|---|
| 100 CFM | 6.05 in | 7 in | 374 fpm |
| 200 CFM | 7.83 in | 8 in | 573 fpm |
| 400 CFM | 10.14 in | 12 in | 509 fpm |
| 800 CFM | 13.14 in | 14 in | 748 fpm |
| 1,000 CFM | 14.29 in | 16 in | 716 fpm |
| 1,200 CFM | 15.30 in | 16 in | 859 fpm |
| 1,600 CFM | 17.05 in | 18 in | 905 fpm |
The calculator flags speeds above 900 feet per minute, which can be noisy in homes. Where a size falls just over a standard size, as 6.05 inches does, a designer may accept the smaller size and slightly higher friction.
Round to rectangular
A rectangular duct carries the same air with the same friction as a round duct of its equivalent diameter, by the Huebscher formula: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25, with sides a and b in inches.
| Rectangular duct | Equivalent round diameter |
|---|---|
| 8 × 10 in | 9.76 in |
| 8 × 14 in | 11.46 in |
| 10 × 12 in | 11.96 in |
The calculator lists the smallest rectangular sizes, at heights of 6, 8, 10 and 12 inches, that match the round size you need.
Trunk lines and branch ducts
The main trunk carries the system's total airflow, and each branch carries one room's share. Room airflow comes from a room-by-room load calculation; the ACCA's Manual D is the industry method for designing the whole duct system, including fittings, which add friction of their own. Use this calculator to size individual runs once you know their airflow.
Flexible duct has more friction than smooth sheet metal, so size it larger. Leaky ducts also waste energy: the DOE puts duct air losses at about 30% of a cooling system's energy; see duct leakage testing.
BTU to CFM: airflow from capacity
A system's capacity is rated in BTU per hour, but ducts are sized by airflow in CFM, so converting one to the other is the step between choosing the equipment and sizing the ducts.
The sensible heat formula
For air at standard conditions, BTU per hour = 1.08 × CFM × temperature difference (°F). The 1.08 comes from the density and specific heat of air, times 60 minutes. Rearranged, CFM = BTU per hour ÷ (1.08 × ΔT).
Cooling: CFM per ton
Air conditioners are designed around a target airflow across the indoor coil, typically about 400 CFM per ton, where one ton is 12,000 BTU per hour:
| System | Cooling capacity | Airflow at 400 CFM per ton |
|---|---|---|
| 2 tons | 24,000 BTU/hr | 800 CFM |
| 3 tons | 36,000 BTU/hr | 1,200 CFM |
| 4 tons | 48,000 BTU/hr | 1,600 CFM |
Humid climates often run lower airflow per ton, so the coil runs colder and removes more moisture; dry climates often run higher, for more sensible cooling.
Heating: temperature rise
A furnace's nameplate lists an allowed temperature-rise range, and the airflow must keep the rise inside it. An 80,000 BTU per hour output needs about 1,481 CFM for a 50°F rise, or 1,058 CFM for a 70°F rise. Too little airflow overheats the heat exchanger; too much delivers lukewarm air.
Frequently asked questions
What size duct do I need for 400 CFM?
About 10.1 inches round at 0.08 inches of water per 100 feet, so a 12-inch duct among the calculator's standard sizes. A 10-inch duct would run slightly above that friction rate.
How many CFM per ton?
About 400 CFM per ton is a typical design airflow; humid climates often use less and dry climates more. The equipment's specifications give its design range.
How do I convert round duct to rectangular?
Use the Huebscher formula above, or the calculator's rectangular list: for example, a 10 × 12 inch duct is equivalent to about a 12-inch round duct (11.96 inches).
What friction rate should I use?
It depends on the pressure your fan can supply after filters, coils and grilles take their share. 0.08 inches of water per 100 feet is a common residential starting point; Manual D sets it for a specific system.
How do I convert BTU to CFM?
For heating or sensible cooling, CFM = BTU per hour ÷ (1.08 × temperature difference). For cooling equipment, multiply tons by about 400 CFM.
How many CFM does a 3-ton AC need?
About 1,200 CFM at 400 CFM per ton.