What size duct do you need for 500 CFM? For a supply trunk carrying 500 CFM, use a 11 inch round (or 14×8, 12×10, 10×12 rectangular), which gives an air velocity of 758 FPM — inside the 700–900 FPM target. That is also the standard airflow for a 1.3 ton system (400 CFM per ton). The calculator below is pre-set to this airflow.
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.
500 CFM Duct Size by Run Type
The same 500 CFM needs a different size depending on where in the system the duct sits — returns and branches run slower, so they must be larger:
| Duct run | Target velocity | Round duct | Actual velocity |
|---|---|---|---|
| Supply trunk | 700–900 FPM | 11 inch | 758 FPM |
| Supply branch | 400–600 FPM | 12 inch | 637 FPM |
| Return trunk | 600–800 FPM | 11 inch | 758 FPM |
| Flexible duct (same run) | 20% lower allowable | 12 inch | 637 FPM |
How 500 CFM Becomes a Duct Size
Duct area = airflow ÷ velocity. At 500 CFM and the 700–900 FPM target for a supply trunk, that works out to a 11 inch round duct running at 758 FPM, with a friction rate of about 0.072 in. w.g. per 100 ft. Rectangular alternatives (14×8, 12×10, 10×12) are sized using the ASHRAE equivalent-diameter formula, so they carry a little more cross-section than the round duct they replace. In flexible duct go up to 12 inch — the ribbed liner adds far more friction than smooth metal.
Frequently Asked Questions
What size duct do I need for 500 CFM? A 11 inch round duct for a supply trunk, or 14×8, 12×10, 10×12 rectangular. That runs at 758 FPM, within the 700–900 FPM target.
What size flex duct for 500 CFM? Go one size up to 12 inch. Flexible duct’s ribbed inner liner creates much more friction than smooth sheet metal, so the same airflow needs a larger diameter.
Is the return duct the same size? No — returns run at lower velocity (600–800 FPM), so 500 CFM of return air needs about 11 inch. Undersized return is the most common airflow fault in homes.
Sizing is half the job — here is what to get right on an air handling unit install.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
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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.