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Calculates friction loss, air velocity (FPM), and CFM per room; generates visual rectangular-to-round duct conversion schematics.
Aerodynamic equivalent equations, velocity acoustics, and static pressure drop solvers
Converts round duct airflow diameters into exact rectangular sheet metal dimensions matching identical friction rates.
Evaluates feet-per-minute (FPM) air speeds against ASHRAE residential quiet standards to prevent turbulence whistling and rumble.
Computes friction pressure losses (in. w.g.) factoring in straight duct runs and fitting equivalent length resistance.
Equal friction duct sizing ensures that round and rectangular shapes deliver identical air volume with matching pressure drops.
Limiting residential main trunk speeds under 800 FPM and branch ducts under 600 FPM eliminates irritating register hiss.
Square-like rectangular ducts (under 2:1 ratio) maximize aerodynamic volume while minimizing sheet metal perimeter friction.
Everything you need to know about HVAC Duct Sizing & Airflow
The Huebscher formula converts rectangular duct dimensions (a x b) into an equivalent round diameter (De) having the exact same friction loss and airflow capacity: De = 1.30 x ((a x b)^0.625) / ((a + b)^0.25).
Standard residential HVAC systems require approximately 400 CFM of airflow per ton of cooling capacity (e.g. a 3-ton AC unit requires 1,200 CFM).
For whisper-quiet residential operation, main supply trunks should stay between 700 and 900 FPM, branch ducts between 500 and 700 FPM, and return air grilles under 500 FPM.
Explore acoustic room planners, pipe flow simulators, and heat sink thermal calculators.
Flags high aspect ratios (>3:1) that restrict blower performance and provides a full table of alternate width/height combinations.
Air velocities exceeding 1,000 FPM in residential ducts generate audible air whistling, register rattling, ductwork rumble, and increased static pressure that strains the blower motor.
Keep the ratio of width to height (W:H) at 2:1 or lower whenever possible. Aspect ratios above 3:1 significantly increase friction loss, sheet metal cost, and thermal heat transfer.
A design friction loss rate of 0.08 inches water column (in. w.g.) per 100 feet of duct is the industry standard for residential supply trunks under ACCA Manual D.
Duct fittings (90° elbows, dampers, register boots) cause turbulence equivalent to several feet of straight duct. This is modeled as Equivalent Length (e.g. a standard 90° elbow adds ~15 to 30 equivalent feet).
Yes! Click 'Export CSV' to download complete engineering tables containing round diameters, rectangular dimensions, actual velocities, and static pressure drops, or click 'Snapshot' for visual schematics.
Calculate the room's sensible heat load BTU/hr (e.g. 2,000 BTU/hr requires ~100 to 120 CFM), then select a 6-inch (100 CFM) or 7-inch (150 CFM) round branch duct operating at 500 to 600 FPM.
Yes! You can switch to 'Commercial Main Supply' to design for higher allowable air velocities up to 1,200–1,500 FPM.
Round duct is the most aerodynamically efficient shape (lowest surface area per CFM, lowest friction). Rectangular duct is used when ceiling joist height is constrained.
No software installation is required. The HVAC Duct Sizing & Airflow CFM Calculator runs client-side in your web browser using HTML5 Canvas and TypeScript.