Duct sizing
What size duct should I run?
Tell me about the run
SUPPLY AIRFLOW
1200CFM
Off the air handler spec sheet
HOW FAST THE AIR MOVES
800FPM
700 to 900 is the sweet spot. Faster gets noisy, slower won't reach the room.
FRICTION RATE
0.08in.wg/100ft
Leave at 0.08 for most supply runs. Only change if your design calls for it.
Run this duct
15.5" round
Run a 15.5 inch round galvanized duct, sized at your friction rate, or pick one of the rectangular sizes below. In flex duct, step up to about 14".
Good airflow
916 FPM, right in the quiet 700-900 range.
If you'd rather run rectangular
| Size (W x H) | Area | Airflow |
|---|---|---|
| 25.5" x 8.5" | 217 sq in | 797 FPM |
| 21.0" x 10.5" | 221 sq in | 784 FPM |
| 18.0" x 12.0" | 216 sq in | 800 FPM |
| 14.5" x 14.5" | 210 sq in | 822 FPM |
All four carry the same 1200 CFM, sized to your velocity target, so their airflow can read differently from the friction-sized round above. Pick whichever fits the chase or joist space.
Field reference only. Confirm against project static, fittings, and local code before fab.