How this calculator works
Moving air through a duct costs pressure, and the size of a duct sets how much. Designers pick a friction rate (a pressure loss per 100 feet of duct, usually 0.10 inches of water) and size the duct so the flow just meets it. The friction chart that HVAC designers use is described by an equation, which this calculator solves for the diameter. A rectangular duct with the same equivalent diameter has the same friction, so we also solve for the other side of a rectangle when you give one side.
Worked example: 200 CFM at the usual 0.10 in wg per 100 ft needs a round duct 7.56 in across, so an 8 in standard duct, which moves the air at 573 fpm. 100 CFM needs 5.8 in (a 6 in duct), 400 CFM needs 9.8 in (10 in) and 1,200 CFM, which is a 3 ton system, needs 14.9 in (16 in). A lower friction rate of 0.08 makes the 200 CFM duct 7.9 in and 0.15 makes it 7.0 in. A rectangular duct 8 in high needs a side of 6.0 in for the 200 CFM, and one 10 in high needs 8.1 in for 400 CFM.
Round duct size by airflow (0.10 in wg per 100 ft)
The diameter needed, the next standard size and the air speed.
| Airflow | Diameter needed | Standard size | Air speed |
|---|---|---|---|
| 50 CFM | 4.5 in | 5 in | 367 fpm |
| 100 CFM | 5.8 in | 6 in | 509 fpm |
| 150 CFM | 6.8 in | 7 in | 561 fpm |
| 200 CFM | 7.6 in | 8 in | 573 fpm |
| 300 CFM | 8.8 in | 9 in | 679 fpm |
| 400 CFM | 9.8 in | 10 in | 733 fpm |
| 600 CFM | 11.5 in | 12 in | 764 fpm |
| 800 CFM | 12.8 in | 14 in | 748 fpm |
| 1000 CFM | 13.9 in | 14 in | 935 fpm |
| 1200 CFM | 14.9 in | 16 in | 859 fpm |
| 1600 CFM | 16.6 in | 18 in | 905 fpm |
Tips before you buy
- A typical rule is about 400 CFM per ton of cooling, so a 3 ton system moves about 1,200 CFM. Share it between the supply runs by the heat each room needs.
- A smaller duct is cheaper but louder, since the air runs faster. Keep branches near 600 to 700 fpm for quiet rooms.
- Elbows, transitions and dampers add friction. A rough way to allow for them is to count each 90 degree elbow as about 10 to 15 ft of straight duct.
- Flexible duct has more friction than rigid duct, and sags and kinks. Keep it short and stretched.
- Seal duct joints with mastic or foil tape rated for ducts, and insulate ducts that run through an attic.
Tools & materials for this project
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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, with air at 509 feet per minute.
What size duct for 400 CFM?
About 9.8 inches round, so a 10 inch duct (733 fpm). That is the airflow of a 1 ton air conditioner.
What size duct for 1,200 CFM?
14.9 inches round, so a 16 inch main duct. That is the flow for a 3 ton system at about 400 CFM per ton.
What is a good friction rate for ducts?
Residential systems are commonly designed at about 0.08 to 0.10 inches of water per 100 feet. A lower rate means a bigger, quieter duct and a higher rate a smaller and noisier one.
How fast should air move in a duct?
About 600 to 700 feet per minute in residential branch ducts and up to about 900 in trunks. A faster flow is noisy and costs more fan power.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.