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Calculate the correct duct size for HVAC systems. Get round and rectangular duct dimensions based on CFM and velocity requirements.
Recommended: 500-900 FPM
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Duct sizing is the process of determining the correct dimensions for HVAC ductwork to deliver the required airflow (CFM) to each room. Properly sized ducts ensure efficient operation, comfort, and low noise levels.
The fundamental relationship between airflow, duct size, and velocity:
CFM = Area (sq ft) × Velocity (FPM)
Area (sq in) = CFM × 144 ÷ Velocity
Round Diameter = √(4 × Area ÷ π)
| Variable | Description | Units |
|---|---|---|
| CFM | Cubic feet per minute of airflow | ft³/min |
| Area | Cross-sectional area of duct | sq in or sq ft |
| Velocity | Air speed through duct | FPM (ft/min) |
Quick reference for round duct sizes at various CFM and velocity combinations:
| Duct Size | Area (sq in) | CFM @ 500 FPM | CFM @ 700 FPM | CFM @ 900 FPM |
|---|---|---|---|---|
| 4” | 12.6 | 44 | 61 | 79 |
| 5” | 19.6 | 68 | 95 | 123 |
| 6” | 28.3 | 98 | 138 | 177 |
| 7” | 38.5 | 134 | 187 | 241 |
| 8” | 50.3 | 175 | 245 | 314 |
| 10” | 78.5 | 273 | 382 | 491 |
| 12” | 113.1 | 393 | 550 | 707 |
| 14” | 153.9 | 535 | 749 | 963 |
| 16” | 201.1 | 698 | 978 | 1,257 |
| 18” | 254.5 | 884 | 1,238 | 1,592 |
Air velocity affects noise, pressure drop, and system efficiency. Lower velocities are quieter but require larger ducts.
| Application | Velocity Range (FPM) | Typical (FPM) | Notes |
|---|---|---|---|
| Main Trunk | 700-1,200 | 900 | Away from living areas |
| Branch Ducts | 500-900 | 700 | To rooms |
| Supply Register | 300-500 | 400 | Minimize noise |
| Return Main | 600-1,000 | 800 | Away from grilles |
| Return Grille | 300-500 | 350 | Keep very low |
| Flex Duct | 400-700 | 500 | Higher friction |
150 CFM required for a bedroom, maximum 700 FPM velocity
Area = 150 × 144 ÷ 700 = 30.9 sq in
Diameter = √(4 × 30.9 ÷ π) = 6.3”
Use 7” round duct
Actual velocity = 150 × 144 ÷ 38.5 = 561 FPM ✓
1,200 CFM total, 900 FPM velocity
Area = 1,200 × 144 ÷ 900 = 192 sq in
Diameter = √(4 × 192 ÷ π) = 15.6”
Use 16” round duct
Or rectangular: 20” × 10” (200 sq in)
400 CFM, max 6” height due to ceiling space
Area needed = 400 × 144 ÷ 700 = 82.3 sq in
With 6” height: Width = 82.3 ÷ 6 = 13.7”
Use 14” × 6” rectangular (84 sq in)
Aspect ratio: 2.3:1 ✓ (under 4:1)
For cooling: CFM = Room sq ft × Ceiling height ÷ 60 × ACH (air changes per hour). Typically 6-8 ACH for residential. Alternatively: CFM = Room BTU load ÷ (1.08 × temperature difference).
The recommended maximum aspect ratio is 4:1 (width to height). Higher ratios increase friction loss and material use. A 2:1 ratio is more efficient than 4:1.
Flex duct has an irregular inner surface that creates more friction than smooth metal duct. It also tends to sag and create restrictions if not properly supported. Limit flex duct runs to 6-8 feet.
Standard design is 400 CFM per ton of cooling. So a 3-ton system needs approximately 1,200 CFM of airflow.
It's the round duct diameter that would have the same friction loss as a rectangular duct. Formula: De = 1.3 × (W×H)^0.625 ÷ (W+H)^0.25. This is important when converting between shapes.