Calculate duct diameter, velocity, and pressure drop for HVAC systems. Size round and rectangular ducts to meet airflow and noise requirements.
The Air Duct Size Calculator determines the correct duct dimensions for any HVAC application based on airflow (CFM), velocity limits, and acceptable pressure drop. Properly sized ducts ensure efficient air delivery, low noise, and minimal energy waste.
Undersized ducts create excessive velocity, noise, and pressure drop — forcing fans to work harder and wasting energy. Oversized ducts waste material and space while potentially causing low-velocity problems like poor air distribution. This calculator finds the optimal balance by computing round duct diameter, equivalent rectangular dimensions, air velocity, and friction loss per 100 feet of duct.
Enter your required CFM and maximum velocity to get instant duct sizing. Compare round vs. rectangular options, check noise criteria compliance, and review the friction loss to verify your design stays within the system's static pressure budget. The result gives you a practical starting point before you size fittings or choose a standard duct size. It also helps you sanity-check whether an existing duct run is undersized before you redesign the system.
Use this calculator when you need a starting duct size that balances airflow, noise, and pressure drop instead of guessing from an old duct run or rule of thumb. It is useful for retrofit planning, branch sizing, and comparing whether round or rectangular duct makes more sense in a tight space.
Duct Area (ft²) = CFM / (Velocity in ft/min). Round Diameter (in) = √(4 × Area / π) × 12. Equivalent Round Diameter = 1.3 × (a×b)^0.625 / (a+b)^0.25. Friction Loss ≈ (0.109136 × Q^1.9) / (D^5.02) (simplified Darcy-Weisbach).
Result: 9.1 inch round duct (use 10" standard)
Area = 400/900 = 0.444 ft². Diameter = √(4×0.444/π)×12 = 9.05". Round up to 10" standard size. Actual velocity at 10" = 733 ft/min. Friction ≈ 0.08 inwg/100ft.
The equal friction method is the most common residential approach: design all duct sections for the same friction loss per unit length (typically 0.08 inwg/100ft). This simplifies balancing because pressure drops are inherently similar. The static regain method, used in large commercial systems, sizes ducts so that the static pressure regain from velocity reduction offsets friction loss at each takeoff.
For residential systems, Manual D from ACCA provides the industry-standard duct design procedure, accounting for available static pressure, total effective length, and equipment specifications.
ASHRAE recommends maximum duct velocities based on noise criteria (NC) ratings. Residential bedrooms: NC 25-30 (velocity under 600 ft/min). Offices: NC 30-35 (under 800 ft/min). Retail spaces: NC 35-40 (under 1,000 ft/min). Industrial: NC 40-50 (under 1,500 ft/min). These limits apply at the point of delivery; main trunk velocities can be higher since they're farther from occupied spaces.
Galvanized steel is the standard for commercial and residential trunk lines. Fiberglass duct board provides built-in insulation. Flexible duct (flex) is used for final branch connections. Each material has different friction factors — galvanized steel is smoothest (lowest friction), while flex duct has significantly higher friction, especially when not fully extended.
Main trunk lines: 700-900 ft/min. Branch runs: 500-700 ft/min. Supply registers: 500-750 ft/min. Return air: 400-600 ft/min. Lower velocities mean less noise but larger ducts.
Use the equivalent diameter formula: D_eq = 1.3 × (a×b)^0.625 / (a+b)^0.25, where a and b are the rectangular sides in inches. A 12"×8" rectangular duct is equivalent to about a 10.7" round duct.
High air velocity is the primary cause. Velocities above 900 ft/min in branch ducts create noticeable noise. Turbulence at fittings, dampers, and transitions also generates noise. Keep main ducts under 1,200 ft/min for quiet operation.
Friction loss is the pressure drop per unit length of duct, typically measured in inches of water gauge per 100 feet (inwg/100ft). Standard residential design uses 0.08 inwg/100ft. Commercial: 0.08-0.15 inwg/100ft.
Rigid metal duct has lower friction loss and is preferred for main runs. Flex duct is convenient for final connections but must be pulled taut — sagging flex duct can have 2-3x the friction loss of rigid duct.
At higher altitudes, air is less dense, requiring higher volume (CFM) to deliver the same mass of air. At 5,000 ft elevation, increase CFM by about 17% compared to sea level. Duct sizes increase accordingly.