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Duct Size Calculator
Size round and rectangular duct from airflow and friction rate, with velocity checks.
Round duct size
10"round
- Velocity at this sizeSupply branch ceiling is 600 FPM
- 550 FPM
- Actual friction at this sizeDesign rate was 0.08 in/100 ft
- 0.053 in/100 ft
- Free area0.545 sq ft
- 78.5 sq in
- Capacity at the design rate81% of this duct's capacity is used
- 372 CFM
Velocity is inside the quiet range for a supply branch
| Rectangular size | Equivalent round | Free area | Velocity | Aspect ratio |
|---|---|---|---|---|
| 10" × 8" | 9.76" | 80 sq in | 540 FPM | 1.3:1 |
| 14" × 6" | 9.80" | 84 sq in | 514 FPM | 2.3:1 |
| Diameter | Capacity | Velocity at capacity | Velocity at your CFM |
|---|---|---|---|
| 8" | 207 CFM | 592 FPM | 859 FPM |
| 9" | 282 CFM | 638 FPM | 679 FPM |
| 10" — selected | 372 CFM | 683 FPM | 550 FPM |
| 12" | 603 CFM | 768 FPM | 382 FPM |
| 14" | 906 CFM | 848 FPM | 281 FPM |
Equivalent rectangular sizes come from the Huebscher relationship, De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Two ducts with the same equivalent round diameter move the same air at the same friction rate, but a flatter duct has more surface area, more heat loss and more sheet metal, so aspect ratios beyond about 4:1 are avoided where framing allows.
What this assumes
- Based on the Darcy-Weisbach friction relationship for galvanized duct at standard air density.
- Default friction rate is 0.08 in. w.c. per 100 feet, typical for residential systems.
- Flexible duct carries significantly higher friction and should be sized one step larger than rigid.
Important
A full duct design requires ACCA Manual D, including total effective length, fitting equivalent lengths and available static pressure from the blower table.
Understanding the result
Velocity drives noise
Residential supply trunks are generally kept under 900 feet per minute and branch runouts under 600 FPM. Above those figures, the duct becomes audible. Commercial systems tolerate higher velocity because ambient noise is higher and duct is usually lined.
Return air is the usual failure point
Undersized returns are the most common duct defect in residential systems. Starved returns raise static pressure, cut airflow across the coil and can freeze an evaporator. Return duct should be sized at least as generously as supply.
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