What size duct does a kitchen range hood need? Exhaust ducts run faster than supply air (1,000–1,500 FPM) because noise matters less and higher speed carries grease out of the run. For a 600 CFM hood, use a 9 inch round duct in smooth rigid metal. Many hood manufacturers quote a minimum size — going one size larger reduces noise and improves performance on long runs.
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.
Range Hood Duct Size by CFM
Smooth rigid metal, exhaust velocity:
| Hood airflow | Round duct | Velocity |
|---|---|---|
| 200 CFM | 5 inch | 1,467 FPM |
| 300 CFM | 7 inch | 1,123 FPM |
| 400 CFM | 7 inch | 1,497 FPM |
| 600 CFM | 9 inch | 1,358 FPM |
| 900 CFM | 11 inch | 1,364 FPM |
| 1,200 CFM | 14 inch | 1,123 FPM |
Range Hood Duct Rules That Matter
Rigid metal only — never flexible duct. Ribbed liners trap grease, which is both a performance and a fire problem. Keep the run as short and straight as possible; every elbow costs the equivalent of several feet of straight duct. Vent to the outside, never into an attic or crawl space. And for powerful hoods (roughly 400 CFM and above in a tight modern home) check whether local code requires make-up air — a hood that extracts more air than the house can replace will backdraft other appliances.
Frequently Asked Questions
What size duct for a 600 CFM range hood? A 9 inch round smooth metal duct, running at 1,358 FPM. Check your hood’s minimum spec and size up on long runs.
Can I use flexible duct for a range hood? No. The ribbed liner traps grease and restricts airflow. Range hood exhaust must be smooth rigid metal, vented outdoors.
Do I need make-up air for a range hood? Often, above roughly 400 CFM in a tightly built home — and many jurisdictions require it. Without make-up air a strong hood can backdraft combustion appliances.
Sizing is half the job — here is what to get right on an air handling unit install.
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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.