RO HVAC Duct Sizer
Professional HVAC duct sizing calculator
Design Inputs
Calculated Duct
Round duct—
Rectangular duct—
Air velocity—FPM
Friction rate—in. w.c. / 100 ft
Airflow & Velocity Calculator
Velocity = CFM ÷ duct area. Required area = CFM ÷ target velocity.
Pressure Drop / Friction Helper
Design friction rate = available static pressure × 100 ÷ total effective length.
Round ↔ Rectangular Size Converter
Equivalent diameter: De = 1.30 × (W×H)^0.625 ÷ (W+H)^0.25.
Common Design Ranges
| Application | Typical velocity | Suggested max |
|---|---|---|
| Residential supply | 700–900 FPM | 900 FPM |
| Supply branch | 500–800 FPM | 800 FPM |
| Return | 400–700 FPM | 700 FPM |
| Light commercial | 900–1500 FPM | 1500 FPM |
Standard Round Sizes
4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36 in
Design Notes
- Use room-by-room design airflow rather than floor-area rules of thumb.
- Keep rectangular aspect ratio preferably at or below 4:1.
- Flex duct generally needs additional allowance for installed roughness and compression.
- Verify final design against applicable ASHRAE, ACCA, SMACNA and local requirements.
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