Smoke that hangs for hours isn’t “normal.” Tests reveal an undersized duct can slash range-hood performance by 50 percent.
The duct size for range hood is the silent force behind quiet air flow, safe temperatures, and a comfortable kitchen environment. Today, I’ll show you the starting point, the science, and the simple steps to pick the right duct size and banish lingering cooking air for good.
Keynote: Duct Size for Range Hood
Match duct to hood power. 0-400 CFM needs 4-6 inches; 401-600 CFM, 6-8 inches; 601-900 CFM, 7-10 inches; 900 + CFM, 8-12 inches. Choose rigid metal. Keep runs short, straight, sealed, and vent outdoors. Proper sizing maximizes airflow, cuts noise, and meets code.
The Hidden Risks of Incorrect Duct Sizing
An undersized air duct strangles the range hood exhaust. Grease stays, odors spread, and indoor air quality plummets. Motors strain, creating louder noise and shortening lifespan. Hot air builds around the cooking area, raising fire risk in both homes and commercial kitchens. Ignoring duct size requirements can also break local building codes, inviting costly re-work.
Unlocking the Power of CFM Ratings
Cubic feet per minute (CFM) shows the volume of air your hood moves. The higher the volume of air, the larger the required duct size.
Use the chart as your quick reference:
| Range Hood CFM | Minimum Duct Size | Ideal Duct Size |
|---|---|---|
| 0–400 | 4 inches | 6 inches |
| 401–600 | 6 inches | 8 inches |
| 601–900 | 7 inches | 8–10 inches |
| 900+ | 8–10 inches | 10–12 inches |
A 30-inch island hood often demands 375 CFM. That figure instantly rules out any minimum size duct below 6 inches.
Always match CFM to duct sizing before range hood installation. Your owner’s manual remains the most important feature guide.
Measuring and Planning Your Duct System
Step 1 – Measure: Record the diameter and duct length of any existing range hood ductwork.
Step 2 – Calculate equivalent length: Each 90-degree elbow adds about 10–25 feet of friction length.
| Round Duct Diameter (inches) | Equivalent Length per 90° Elbow (feet) |
|---|---|
| 4 | ~10 |
| 6 | ~10–15 |
| 8 | ~10–15 |
| 10 | ~10–20 |
| 12 | ~10–25 |
Keep total duct run under 30 feet. Fewer bends equal less air flow resistance and maximum efficiency. Hot air rises, so vertical venting is preferred when the exterior of your home allows it.
Duct Materials and Shapes: What Works Best?
Round ducts create less air resistance than rectangular duct systems, moving cooking air with 20–30 percent more efficiency.
Rigid stainless steel or galvanized metal duct is the gold standard. A smooth metal duct resists grease, meets building codes, and lasts decades.
Flexible duct seems like an easy option, yet its ridges trap grease and fail code checks. Rectangular ducts fit tight spots but require larger dimensions to match the same airflow.
Installation Secrets for Peak Performance
Short, straight runs give optimal performance. Aim for no more than two 90-degree elbow turns in one duct system.
Seal every joint with UL-listed foil tape, not common “duct” tape. Proper sealing prevents leaks and protects indoor air systems from grease.
Vent outside of the home, never into attics or ceilings. Local building codes insist on metal ducts that terminate outdoors (Source: 2024 IRC).
7 Costly Mistakes Homeowners Make
- Ignoring CFM ratings when choosing hood types.
- Reusing undersized four-inch ducts during range hood installation.
- Installing PVC or flex ducts instead of metal.
- Skipping code checks and inspection labels.
- Forgetting regular filter cleaning, which blocks air flow.
- DIY reroutes during complex installation scenarios without a pro.
- Venting indoors to “save money,” the only option that truly risks fire.
Troubleshooting: Signs Your Duct Is Holding You Back
Weak suction, excessive noise, or stubborn odors show airflow resistance.
First, clean filters and the range hood ductwork. Replace any crushed round ducts or kinked rectangular runs.
If problems persist, upgrade to the appropriate duct size or call a certified installer for complex installation scenarios.
Ductless Range Hoods: A Viable Alternative?
These hood types recirculate air through charcoal filters. They are the right duct size workaround when venting outside isn’t possible.
Pros include quick setup and zero exterior holes. Cons include lower indoor air quality during heavy frying and frequent filter swaps. For island hoods with high BTUs, ductless models remain a last resort.
Conclusion
Choosing the correct duct size for range hood follows six simple steps: confirm CFM, match duct size requirements with the chart, plan a short vertical venting route, use rigid metal, seal tightly, and verify code.
Stick to those rules and you’ll enjoy quieter cooking, cleaner air, and a kitchen range hood that reaches optimal performance every meal.
What Size Duct for Range Hood (FAQs)
Can I shrink duct diameter mid-run?
No. Minimum size duct reductions boost airflow resistance and void warranties.
Are periscope ducts code-compliant?
Rarely. They trap grease and break range hood ventilation system rules.
Do I need insulation on metal ducts?
Yes, when longer duct runs pass through unheated spaces to stop condensation.
Is horizontal venting allowed?
Yes, but vertical venting enjoys less air resistance and better draft.
Katie Lee has over 20 years of experience in the kitchen. She helps homeowners find the right appliances for their needs to sets up a perfect kitchen system. She also shares helpful tips and tricks for optimizing appliance performance.