Ductulator: size a duct from CFM
This ductulator gives you the round duct size, the matching rectangle sizes and the air speed for any airflow in CFM.
How a duct is sized
Air pushes against the walls of a duct. That drag is called friction. A small duct means high air speed and a lot of friction. That is loud and hard on the fan. A big duct costs more and takes room. Sizing finds the middle.
The equal friction method picks the duct so every foot of duct has the same friction. You give the airflow and the friction rate, and the formula gives the diameter.
The formula assumes smooth metal. For rougher duct we divide the friction rate by a roughness factor: 1.08 for duct board and 1.4 for flex. That gives a bigger diameter.
Speed in the duct
Many designers keep branch ducts near 600 feet per minute and trunks at 700 to 900. Over 900 gets noisy in a home. Under 300 the duct may be bigger than it needs to be. The speed check in the result follows that.
Common round sizes at 0.08 friction
These sizes use smooth metal duct and a friction rate of 0.08. They were figured when the page was built.
| CFM | Exact round | Standard round | Air speed |
|---|---|---|---|
| 50 | 4.7 in | 5 in | 367 ft/min |
| 75 | 5.5 in | 6 in | 382 ft/min |
| 100 | 6.1 in | 6 in | 509 ft/min |
| 150 | 7.1 in | 7 in | 561 ft/min |
| 200 | 7.9 in | 8 in | 573 ft/min |
| 300 | 9.2 in | 9 in | 679 ft/min |
| 400 | 10.3 in | 10 in | 733 ft/min |
| 600 | 12.0 in | 12 in | 764 ft/min |
| 800 | 13.4 in | 14 in | 748 ft/min |
| 1000 | 14.5 in | 14 in | 935 ft/min |
| 1200 | 15.6 in | 16 in | 859 ft/min |
| 1600 | 17.4 in | 18 in | 905 ft/min |
| 2000 | 18.9 in | 18 in | 1,132 ft/min |
Standard round sizes are 3 to 10 inches in steps of 1, then 12 to 30 in steps of 2 (14, 16, 18 and so on), and 36. We pick the smallest stock size that is no more than 5 percent under the exact size. A 10.3 in answer becomes a 10 in duct. The friction in that size is a little over the target, and the result shows it.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
What friction rate should I use for home ducts?
Many home designs use about 0.08 to 0.1 inches of water per 100 feet. Your fan and the total length of duct decide the real number. A longer run needs a lower rate.
How many CFM does each room need?
It depends on the room load. A common start is about 400 CFM for each ton of cooling, shared out by room. The app splits the load by room and gives CFM and a round duct size for each one.
Why does flex duct need a bigger size?
The inside of flex is rough, so air drags more. To keep the same friction, flex needs a larger diameter. Here, duct board counts as 8 percent rougher than metal and flex as 40 percent rougher.
What is the limit on rectangle shape?
Long, thin rectangles lose more energy than square ones. This page shows shapes up to 4 to 1. Squarer is better.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.