Duct Sizing Calculator
Calculate duct dimensions for optimal airflow
Static Pressure Calculation (Optional)
Recommended Duct Size
Static Pressure Drop
Per 100 ft duct
Incl. fittings
Formulas:
• Area = Airflow ÷ Velocity
• Round: Diameter = √(4 × Area ÷ π)
• Equivalent Diameter = 1.3 × (W × H)^0.625 ÷ (W + H)^0.25
Recommended Duct Velocities
| Duct Location | Residential (FPM) | Commercial (FPM) | m/s |
|---|---|---|---|
| Main Supply Duct | 700-900 | 1000-1300 | 3.5-6.5 |
| Branch Supply Duct | 500-700 | 600-900 | 2.5-4.5 |
| Main Return Duct | 600-800 | 800-1000 | 3.0-5.0 |
| Branch Return Duct | 400-600 | 500-800 | 2.0-4.0 |
| Supply Grille Face | 300-500 | 400-600 | 1.5-3.0 |
| Return Grille Face | 300-500 | 400-500 | 1.5-2.5 |
Noise Considerations
Higher velocities = More noise. For quiet spaces (bedrooms, libraries), use lower end of ranges. Supply velocities above 900 FPM may cause noticeable noise.
Unit Conversions
1 CFM = 1.699 m³/h = 0.472 L/s
1 FPM = 0.00508 m/s
1 inch = 25.4 mm
Standard Round Duct Sizes
| Diameter (in) | Diameter (mm) | Area (ft²) | CFM @ 700 FPM | CFM @ 900 FPM |
|---|---|---|---|---|
| 4" | 100 | 0.087 | 61 | 78 |
| 5" | 125 | 0.136 | 95 | 122 |
| 6" | 150 | 0.196 | 137 | 176 |
| 7" | 175 | 0.267 | 187 | 240 |
| 8" | 200 | 0.349 | 244 | 314 |
| 10" | 250 | 0.545 | 382 | 491 |
| 12" | 300 | 0.785 | 550 | 707 |
| 14" | 350 | 1.069 | 748 | 962 |
| 16" | 400 | 1.396 | 977 | 1257 |
| 18" | 450 | 1.767 | 1237 | 1590 |
Static Pressure Reference
Typical Equipment External Static Pressure
| Equipment Type | Available ESP |
|---|---|
| Residential Furnace (Low) | 0.3-0.5 in.w.c. |
| Residential Furnace (Standard) | 0.5-0.8 in.w.c. |
| Residential Air Handler | 0.5-1.0 in.w.c. |
| Commercial Rooftop Unit | 1.0-2.0 in.w.c. |
| Commercial AHU | 2.0-4.0 in.w.c. |
Fitting Equivalent Lengths (Round Duct)
| Fitting | Equiv. Length |
|---|---|
| 90° Elbow (r/D=1.5) | 10 ft (3m) |
| 90° Elbow (r/D=1.0) | 15 ft (4.5m) |
| 45° Elbow | 5 ft (1.5m) |
| Supply Register | 50-100 ft (15-30m) |
| Return Grille | 30-60 ft (9-18m) |
| Filter (clean) | 0.1-0.2 in.w.c. |
Friction Rate Design Guide
Residential: 0.08-0.10 in.w.c. per 100 ft
Commercial: 0.08-0.15 in.w.c. per 100 ft
Higher friction = smaller ducts but more fan energy
Flex Duct Considerations
Flex duct has 3-5× higher friction than sheet metal.
Keep runs short and straight. Avoid tight bends.
Always fully extend - compressed flex adds significant resistance.