HVAC Duct Sizing Calculator
Enter the airflow you need and the friction rate for your system to get the minimum duct diameter, air velocity, and noise rating. Add multiple branch runs for a complete duct schedule. Nothing uploaded.
Duct size
Rectangular equivalents
Branch run schedule
| Run | CFM | Diameter | Velocity | Noise |
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Learn more: HVAC duct sizing and friction rate calculation
Friction rate and pressure drop - why duct diameter matters
Friction rate is the pressure loss per 100 feet of duct, measured in inches of water gauge (in.w.g.). The standard residential design rate is 0.10 in.w.g. per 100 feet. As air moves through a duct, friction converts kinetic energy into heat, requiring the air handler fan to work harder. Undersized ducts increase both friction losses and air velocity, forcing the fan to draw more power and shortening equipment lifespan. Larger ducts reduce friction but are more expensive and harder to fit. The calculator helps you find the optimum balance.
Air velocity and noise - the ASHRAE limits and what they mean
Air velocity in a duct is measured in feet per minute (FPM). The ASHRAE standard for residential supply ducts is a maximum of 900 FPM to avoid noise. At higher velocities, you get audible whooshing and hissing sounds at vents. Below 500 FPM, the duct is oversized and wastes material. The calculator rates your result as Acceptable (600-900 FPM), Noisy (over 900 FPM), or Oversized (under 500 FPM) to guide your sizing decisions. Branch runs in different parts of the system may have different optimal velocities - your office supply duct can be higher (less sensitive to noise) than a bedroom supply.
Round versus rectangular duct and flex duct corrections
Round ducts offer the lowest friction for a given flow rate and are most efficient. Rectangular ducts are often easier to conceal in walls and crawl spaces but require larger cross-sectional area to carry the same CFM. The hydraulic diameter formula converts between them. Flexible duct (insulated flex hose) has higher internal friction than rigid sheet metal, so the calculator applies a 15% size increase when flex is selected to account for this penalty. Use flex only where needed for bends and short runs; maintain rigid duct for main trunks.
FAQ
What is CFM and where does it come from?
CFM (cubic feet per minute) is the volume of air your system must deliver. It comes from your HVAC design load calculation (Manual J) or your Manual D duct sizing standard. A typical residential supply run to a room might be 50-150 CFM depending on room size and climate.
Can I mix round and rectangular ducts?
Yes. Transitions between round and rectangular are common at supply plenums and room drops. Size the transition piece to match the calculated equivalent diameter to avoid velocity changes that cause noise and pressure loss.
Why does flex duct get a 15% size increase?
The corrugated interior of flexible duct creates more surface area and friction than smooth rigid ductwork. The 15% penalty is an industry-standard correction factor to account for this higher friction and ensure velocity targets are still met.