Duct Calculator
Correctly sized ductwork delivers design airflow to every register without noise, pressure loss, or duct damage. This HVAC duct calculator takes your CFM requirement and application (supply trunk, branch, or return) and returns round duct diameter, equivalent rectangular size, velocity, and pressure drop using ACCA Manual D simplified methodology with a 0.08 in wc/100 ft friction target.
Duct Sizing Formulas
The velocity method (Manual D standard for residential):
- Cross-sectional area (sq ft) = CFM / Velocity (FPM)
- Round diameter (in) = sqrt((4 x Area in sq in) / pi)
For 400 CFM at 700 FPM target velocity:
- Area = 400 / 700 = 0.571 sq ft = 82.3 sq in
- Diameter = sqrt(4 x 82.3 / pi) = 10.2 inches, rounded up to 10-inch round duct.
Duct Sizing Chart (CFM to Round Duct)
| CFM | Supply Trunk (850 FPM) | Supply Branch (650 FPM) | Return (750 FPM) |
|---|---|---|---|
| 100 | 5" | 6" | 5" |
| 200 | 7" | 8" | 7" |
| 300 | 8" | 9" | 9" |
| 400 | 9" | 10" | 10" |
| 600 | 11" | 13" | 12" |
| 800 | 13" | 15" | 14" |
| 1000 | 15" | 17" | 16" |
Flex Duct CFM Chart
Flex duct has internal ribbing that increases friction, so it carries about 10-15% less air than smooth sheet metal at the same diameter:
| Flex Duct Diameter | Max CFM (0.08 in wc/100 ft) |
|---|---|
| 4" | 35 CFM |
| 5" | 60 CFM |
| 6" | 100 CFM |
| 7" | 160 CFM |
| 8" | 230 CFM |
| 10" | 425 CFM |
| 12" | 700 CFM |
| 14" | 1,000 CFM |
| 16" | 1,400 CFM |
Flex duct is cheaper and easier to install but should only be used on branches, kept short (under 10 feet), and pulled tight to avoid excessive pressure loss.
Recommended Duct Velocities
- Supply trunk (main): 800-900 FPM. Higher velocities work but increase noise.
- Supply branch: 600-700 FPM. Keep branches slower to reduce register noise.
- Return trunk: 700-800 FPM.
- Return branch: 500-600 FPM.
- Fresh air intake: 400-500 FPM.
Commercial systems tolerate higher velocities (1,000-1,500 FPM in trunks) because duct runs are shorter relative to total airflow. Residential should stay below 900 FPM in occupied spaces to avoid noise complaints.
Friction Rate
Manual D targets 0.08-0.10 inches of water column per 100 feet of equivalent duct length. The equivalent length accounts for elbows, takeoffs, and transitions (each adds 10-50 feet of "equivalent" straight duct). Multiply actual run length by 1.5-2x to get equivalent length for rough calculations.