Chart
CFM to duct size chart
Start from the airflow you need to move and read off the round duct size, then carry it into the console for the rectangular and flat-oval equivalents. Look it up by target velocity or by friction rate — the two ways designers size duct.
Enter an airflow and a target to read the required round diameter and the nearest standard size.
| CFM | @ 0.08 f.r. | @ 0.10 f.r. | @ 700 fpm | @ 900 fpm |
|---|---|---|---|---|
| 100 | 7" | 6" | 6" | 5" |
| 150 | 8" | 7" | 7" | 6" |
| 200 | 8" | 8" | 8" | 7" |
| 300 | 10" | 9" | 9" | 8" |
| 400 | 11" | 10" | 11" | 10" |
| 600 | 12" | 12" | 13" | 12" |
| 800 | 14" | 13" | 16" | 13" |
| 1,000 | 16" | 14" | 18" | 16" |
| 1,200 | 16" | 16" | 18" | 16" |
| 1,600 | 18" | 18" | 22" | 20" |
| 2,000 | 20" | 18" | 24" | 22" |
Higher friction rates and faster target velocities pack the same air into a smaller duct. The lookup above gives the exact diameter for any airflow; the console adds the rectangular and flat-oval equivalents.
Reading the chart
Sizing by velocity is the quickest path: the required round diameter is
D = √(183.3 · CFM / velocity)
(diameter in inches, velocity in feet per minute), then you round up to the
nearest standard duct size. Sizing by friction rate is more rigorous — it solves
the round diameter that loses exactly your chosen pressure per 100 feet — and is
the method a full design uses.
Either way, the right target depends on the application: residential supply usually aims for roughly 700–900 feet per minute or a 0.08–0.10 friction rate, while light commercial runs faster. The velocity chart and friction rate chart cover each in depth, and the console adds the rectangular and flat-oval equivalents plus the safe-size verdict.