What size supply duct do you need? At 1,200 CFM, the main supply trunk should be 16 inch round (or 30×8, 24×10, 20×12 rectangular), running at 859 FPM — within the 700–900 FPM target for trunks. Individual branch runs to each room carry far less air (typically 100–200 CFM) and are sized slower to keep registers quiet. The calculator below is pre-set to a supply trunk.
HVAC Airflow: Static Pressure, CFM, and Friction Rates
Correct duct sizing is the most ignored aspect of HVAC design, yet it dictates the performance of your entire system. If your ducts are too small, airflow (CFM – Cubic Feet per Minute) is restricted, causing the blower motor to overwork, increasing static pressure, and eventually freezing the evaporator coil. When calculating duct size, you must differentiate between rigid metal ducts (smooth interior, lower friction rate) and flexible ductwork (higher friction rate). The calculator below uses standard Manual D principles to help you find the correct trunk and branch dimensions for optimal, quiet airflow.
🌬️ What Size Duct Do You Need for Your Airflow?
Convert CFM into the right round duct diameter and rectangular equivalent, with the correct air velocity for supply trunks, branches, returns and exhaust. Enter CFM directly, or derive it from your AC tonnage or house size — and it adjusts for flexible vs rigid duct.
🔢1. How Do You Want to Enter Airflow?
Enter CFM directly if you know it, or let the calculator derive it from AC tonnage (400 CFM per ton) or from house floor area.
💨2. Airflow
Range: 50 – 4,000 CFM. A single room register is typically 100–200 CFM.
🧭3. Which Part of the Duct System?
Main trunks run faster (700–900 FPM) because they carry the whole system's air. Branches run slower (400–600 FPM) so air doesn't whistle at the registers.
🔩4. Duct Material
Flexible duct has a ribbed inner liner that creates far more friction than smooth sheet metal, so it must be sized one size larger for the same airflow. Keep flex runs short, pulled taut and free of sharp bends.
Supply Trunk vs Branch Sizing
Same 1,200 CFM through different parts of the system:
| Duct run | Target velocity | Round duct | Actual velocity |
|---|---|---|---|
| Supply trunk | 700–900 FPM | 16 inch | 859 FPM |
| Supply branch | 400–600 FPM | 18 inch | 679 FPM |
| Return trunk | 600–800 FPM | 18 inch | 679 FPM |
| Flexible duct (same run) | 20% lower allowable | 18 inch | 679 FPM |
Trunks, Branches and Register Noise
The reason trunks and branches use different velocities is noise. A trunk runs inside a basement, attic or chase where 800 FPM is inaudible. A branch terminates at a register beside someone’s head — above about 600 FPM you start to hear air. That is why a branch carrying 150 CFM uses a 6 inch duct rather than the 5 inch a trunk-speed calculation would allow. Size each room’s branch to its own CFM, not to a fraction of the trunk.
Frequently Asked Questions
What size supply duct for 1,200 CFM? A 16 inch round trunk (or 30×8, 24×10, 20×12), at 859 FPM.
What size branch duct for each room? Size it to the room’s own airflow — typically 100–200 CFM, which is a 6 to 8 inch branch at branch velocity (400–600 FPM).
Should supply and return be the same size? No. Returns use lower velocity, so they end up larger for the same CFM — around 18 inch here versus 16 inch supply.
Once the duct is sized, read our AHU installation guide.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.