14" Round Duct at 75 CFM — Velocity & Friction Calculator
How This Was Calculated
Velocity = CFM / Cross-sectional area. Area = π × (d/2)². Friction calculated via modified Darcy-Weisbach for HVAC duct (SMACNA method).
- V
- Velocity: 70 FPM
- Q
- Airflow: 75 CFM
- d
- Diameter: 14 in
- A
- Cross-sectional area: 1.069 ft²
Important Considerations
Total system static pressure = supply duct friction + return duct friction + coil pressure drop + filter pressure drop + grille/diffuser pressure drop. Typical residential air handler rated external static pressure: 0.5 in. w.g. (budget-grade) to 0.8 in. w.g. (high-efficiency ECM). A common budget allocation: 0.1 in. supply duct, 0.1 in. return duct, 0.15–0.2 in. coil, 0.1 in. filter, leaving margin. Exceeding rated ESP causes reduced airflow and efficiency.
Flex duct (spiral wire inner liner with insulation wrap) has 15–20% higher friction than equivalent hard duct due to its corrugated interior. The friction rate shown assumes smooth galvanized steel. For flex duct, multiply effective length by 1.15–1.25 or use ACCA Manual D flex duct correction factors. Hard duct (galvanized steel, aluminum, or duct board) is preferred for trunk lines. ACCA Manual D and SMACNA recommend limiting flex duct runs to 14 ft maximum with no more than one 90° bend at full diameter.
For 14" ducts, spiral galvanized steel is the industry standard. Large-diameter ductwork requires rigid construction to maintain round shape and minimize pressure loss. Fittings (elbows, transitions, takeoffs) are major pressure drop sources — use sweep elbows over mitered elbows when possible and follow SMACNA fitting recommendations. Seal all joints with mastic or UL 181-rated foil tape; duct tape alone is not approved for sealed duct systems.
Duct sizing uses the equal friction method per SMACNA HVAC Duct Construction Standards (4th Edition) and ASHRAE Fundamentals Chapter 21 (Duct Design). Friction factors assume galvanized steel with absolute roughness of 0.0003 ft. Noise velocity limits per SMACNA Table 5-1.