Flexible duct needs to be one size larger than rigid sheet metal for the same airflow, because its ribbed inner liner creates far more friction. At 400 CFM, for example, a rigid duct of 10 inch does the job — but in flex you need 11 inch. The chart below covers 100 to 2,000 CFM.
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.
Flex Duct Size Chart by CFM
Rigid vs flexible duct for the same airflow:
| Airflow | Rigid metal | Flex duct | Flex velocity |
|---|---|---|---|
| 100 CFM | 6 inch | 6 inch | 509 FPM |
| 200 CFM | 8 inch | 9 inch | 453 FPM |
| 400 CFM | 10 inch | 11 inch | 606 FPM |
| 600 CFM | 12 inch | 14 inch | 561 FPM |
| 800 CFM | 14 inch | 16 inch | 573 FPM |
| 1,000 CFM | 16 inch | 16 inch | 716 FPM |
| 1,200 CFM | 16 inch | 18 inch | 679 FPM |
| 1,600 CFM | 20 inch | 22 inch | 606 FPM |
| 2,000 CFM | 22 inch | 24 inch | 637 FPM |
Installing Flex Duct Properly
Sizing up is only half the job. A flexible duct that is compressed, kinked or sagging between joists can lose more airflow than a whole extra size gains. Pull every run taut, support it every 4–5 feet, keep bends gentle and runs as short as possible. Use rigid sheet metal for main trunks and never use flex for a range-hood exhaust — the ribs trap grease and become a fire risk.
Frequently Asked Questions
How much bigger does flex duct need to be? One nominal size, roughly +1 inch. At 400 CFM a 10 inch rigid duct becomes 11 inch in flex.
Why does flex duct need to be larger? Its ribbed inner liner creates far more friction than smooth galvanised metal, so air loses more pressure over the same distance. Sizing up restores the airflow.
Can I use flex duct for a main trunk? It is not recommended. Trunks carry the whole system’s air, where friction losses matter most — use rigid sheet metal for trunks and reserve flex for short branch runs.
Related reading: how to install an air handling unit.
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.