Your range hood is only as good as the duct behind it. You can spend $1,500 on a premium hood with serious airflow, but if it is pushing air through an undersized duct, you are choking the performance before it even starts. The result is louder operation, weaker suction at the cooking surface, and grease buildup inside ductwork that should be staying clean. This guide covers how to choose the right duct size for your range hood, when to go with 6-inch versus 8-inch ductwork, and how to plan a duct run that actually lets your hood do its job.
Why Duct Size Matters More Than You Think
Think of your ductwork as the exhaust pipe of your kitchen ventilation system. Too narrow, and air backs up. The fan motor works harder, noise increases, and the hood pulls less grease and smoke from your cooktop. Too wide (rare, but it happens), and air velocity drops so low that grease settles inside the duct instead of being carried out.
The relationship is simple physics: air volume (measured in CFM) needs a matching cross-sectional area to flow at the right speed. When the duct is too small for the CFM your hood produces, static pressure builds up inside the system. That pressure forces the motor to strain, which is why an undersized duct often turns a quiet hood into a noisy one.
This is also why ductwork mistakes are the single most common reason range hoods underperform. The hood itself is fine. The duct is the bottleneck.
Range Hood Duct Size Chart: CFM to Duct Diameter
Every range hood manufacturer specifies a minimum duct size. But if you are planning a longer run or adding elbows, you may need to size up. Here is the standard guidance based on CFM output:
| Hood CFM Rating | Minimum Duct Diameter (Round) | Equivalent Rectangular | Best For |
|---|---|---|---|
| Up to 400 CFM | 4 inches | 3.25" × 10" | Microwave hoods, low-output under-cabinet hoods |
| 400–600 CFM | 6 inches | 3.25" × 14" | Standard 30-inch wall-mount hoods |
| 600–900 CFM | 8 inches | 7" × 10" | High-performance 30"–36" hoods |
| 900–1,200 CFM | 8–10 inches | 8" × 12" or larger | Professional-grade hoods, heavy cooking |
| 1,200+ CFM | 10–12 inches | Custom sizing | Commercial-style residential hoods |
These numbers come from the Home Ventilating Institute (HVI) airflow guidelines, which are the industry standard for residential ventilation sizing. Your local building code may add requirements on top of these — particularly around make-up air for hoods rated above 400 CFM.
A Note on Air Velocity vs. CFM
CFM tells you how much air moves per minute. But what actually captures grease and smoke at the cooktop is air velocity — how fast that air moves across the filters or intake. A hood rated at 600 CFM with a 6-inch duct will push air faster through the duct than the same 600 CFM through an 8-inch duct. That is fine inside the duct, but the capture velocity at the hood opening is what matters for cooking performance. This is why some manufacturers, including Arspura, spec air velocity (measured in meters per second) as a performance metric alongside CFM.
6-Inch vs. 8-Inch Duct: Which One Do You Need?
This is the most common decision homeowners face. Most residential range hoods ship with either a 6-inch or 8-inch outlet, and the choice between them is not arbitrary.
When 6-Inch Ductwork Works
- Hood CFM is 600 or below. A 6-inch round duct handles up to about 600 CFM before air velocity gets excessive and noise becomes a problem.
- Short, straight duct run. If your duct path is under 15 feet total equivalent length (more on that below) with one elbow or fewer, 6-inch is adequate.
- Replacing an existing 6-inch run. If your home already has a 6-inch duct through the wall or roof and you are swapping hoods, there is no reason to tear it out unless your new hood demands more.
- Budget-conscious installations. Six-inch duct fittings, wall caps, and roof caps are cheaper and more widely available than 8-inch equivalents.
When You Should Step Up to 8-Inch
- Hood CFM exceeds 600. Pushing 700-900 CFM through a 6-inch duct creates turbulence and significantly increases noise. An 8-inch duct gives the air more room and keeps the system quieter.
- Long duct runs. If your total equivalent duct length exceeds 20 feet, sizing up reduces static pressure drop across the run.
- Multiple elbows. Each 90-degree elbow adds roughly 5–8 feet of equivalent length. Two elbows on a 10-foot run effectively makes it a 20–26 foot run. That is 8-inch territory for a 600+ CFM hood.
- Ceiling-vented installations. Vertical runs through the ceiling and roof are inherently longer and harder to service. Going with 8-inch ductwork on a ceiling-vented installation gives you a performance margin that pays off over the life of the hood.
- Heavy cooking styles. If you regularly do high-heat wok cooking, deep frying, or other grease-heavy cooking, the extra duct capacity helps your hood evacuate smoke and particulates faster.
Rule of thumb: When in doubt, go one size up. An 8-inch duct on a 600 CFM hood will never hurt performance, but a 6-inch duct on a 700 CFM hood will always limit it.
How to Calculate Total Equivalent Duct Length
Straight duct length is only part of the equation. Every fitting in the run adds resistance, which is expressed as "equivalent length." Here is how common fittings translate:
| Duct Fitting | Equivalent Length Added |
|---|---|
| 90-degree elbow | +5 to 8 feet |
| 45-degree elbow | +2.5 to 4 feet |
| Wall cap or roof cap (with damper) | +2 to 3 feet |
| Transition from round to rectangular | +1 to 2 feet |
| Inline backdraft damper | +1 to 2 feet |
To calculate your total equivalent length: measure the straight duct distance from the hood outlet to the exterior termination point, then add the equivalent length for every fitting in the path.
Example: You have a wall-mount hood with 8 feet of straight duct, one 90-degree elbow, and a wall cap. Your total equivalent length is 8 + 6 + 2.5 = 16.5 feet. For a 600 CFM hood, a 6-inch duct handles this fine. For a 750+ CFM hood, you should go with 8-inch.
Most manufacturers recommend keeping total equivalent length under 30 feet. Beyond that, performance drops noticeably regardless of duct size. If your planned run exceeds 30 feet, reconsider the venting path before committing to the installation. The International Residential Code (IRC) Section M1504 provides maximum duct length tables based on duct size and fitting count — your inspector may reference these.
Round vs. Rectangular Duct: What Performs Better?
Round duct is almost always the better choice for range hood venting. It offers lower air resistance per foot than rectangular duct because air flows more smoothly through a circular cross-section with no corners to create turbulence. Round duct is also easier to seal and less prone to grease accumulation at joints.
Rectangular duct has its place when you need to fit ductwork into a tight space — between joists, inside a soffit, or behind a cabinet. The tradeoff is higher resistance and slightly more noise. If you must use rectangular duct, size it to match or slightly exceed the cross-sectional area of the round equivalent (check the chart above).
One common mistake: using flexible duct for the entire run. Flex duct has a corrugated interior that creates significant turbulence and grease trapping. It is acceptable for short transitions (under 2 feet) between the hood collar and the rigid duct, but it should never be the primary duct material. If your current setup uses flex duct for a long run, that is one of the ductwork mistakes worth fixing.
Duct Sizing for Common Installation Scenarios
Here are the most common kitchen setups and the duct sizing that works best for each:
Wall-Mount Hood, Straight Through the Wall
This is the simplest and most efficient setup. The duct goes straight back from the hood, through the wall, and terminates with a wall cap. Total equivalent length is typically 3–8 feet. A 6-inch duct works for hoods up to 600 CFM. An 8-inch duct is appropriate for anything above that. If the gap between your hood and the ceiling needs covering, a chimney extension or trim piece handles the cosmetic side.
Wall-Mount Hood, Up Through the Ceiling and Roof
Vertical duct runs are longer and involve at least one 90-degree elbow where the duct transitions from horizontal to vertical (or vice versa). Total equivalent length often lands between 15 and 25 feet. For this setup, 8-inch duct is the safer choice for any hood rated above 500 CFM. Longer vertical runs also benefit from smooth interior walls (rigid metal) to keep grease from accumulating. See the full breakdown in our ceiling venting guide.
Hood Replacement With Existing Ductwork
If you are replacing an old range hood and keeping the existing duct, check that the duct size meets the minimum requirement for your new hood. Many older homes have 4-inch or 5-inch ducts that were adequate for a 200 CFM microwave hood but are completely undersized for a modern range hood. If the existing duct is too small, you have two options: replace the duct run (more work but better results) or use a reducer fitting (easier but caps your performance).
Using a Duct Extension Connector
When the distance between your range hood and the existing ductwork does not line up cleanly, a connector kit bridges the gap. Arspura offers a Duct Extension Connector Kit designed for exactly this scenario — it maintains the correct duct diameter through the transition so you do not lose airflow to a mismatched joint. This is especially useful in ceiling-vented setups where the chimney cover needs to extend higher than standard.
Common Duct Sizing Mistakes to Avoid
Duct sizing errors are frustrating because they are invisible once the drywall goes back up. Here are the ones contractors and homeowners make most often:
- Reducing duct size to fit through a tight space. A 6-inch to 4-inch reducer in the middle of a run creates a choke point that limits the entire system. If the path is tight, use rectangular duct at the correct cross-sectional area instead.
- Using the wrong duct material. Flexible aluminum duct is convenient but kills performance on runs longer than a couple of feet. Use rigid galvanized steel or aluminum for the primary run.
- Ignoring make-up air requirements. Hoods rated above 400 CFM may require a make-up air system per the IRC Section M1503.6. This does not change duct size, but it affects how the system functions and must be planned during the duct layout phase.
- Too many elbows. Three or more 90-degree elbows in a single run can double your effective duct length. Redesign the path to eliminate at least one before finalizing the installation.
- Terminating inside the attic or soffit. The duct must exit the building envelope. Venting into an attic creates moisture problems, fire hazards, and code violations. Always terminate with an exterior wall cap or roof cap.
If you suspect your existing ductwork has one of these issues, our guide on how to identify and fix ductwork mistakes walks through each scenario with solutions.
Does Your Range Hood Installation Affect Duct Size Choice?
Yes. How the hood gets installed determines the duct path, and the duct path determines the right size. This is why it is important to plan ductwork and installation together rather than treating them as separate decisions.
If you are working with a professional installer, they should assess the duct path during the site visit and recommend the correct size. If you are doing the ductwork yourself or hiring a general contractor, bring the hood's spec sheet (especially CFM rating and recommended duct size) to the planning conversation.
Arspura range hoods ship with a 6-inch duct collar and include FREE professional installation, which means the installer handles duct sizing, routing, and connection as part of the service. This eliminates the guesswork and ensures the ductwork matches the hood's performance specs. For installations that need a longer duct run, the Duct Extension Connector Kit is available as an accessory. Learn more about what the full installation process costs when you are doing it independently versus having it included.
Frequently Asked Questions
Can I use a 4-inch duct for my range hood?
Only if your hood is rated at 300 CFM or below, which typically means a microwave-style exhaust fan or a very small under-cabinet hood. Most modern range hoods require a minimum of 6 inches. Check your hood's installation manual for the manufacturer's minimum duct size before committing to a 4-inch run. Using an undersized duct voids many warranties and will always result in higher noise and lower capture performance.
What happens if my duct is too small for my range hood?
Three things happen. First, the fan motor works harder than it was designed to, which shortens its lifespan. Second, noise increases — sometimes dramatically — because air is being forced through a restriction. Third, the hood loses effective suction at the cooking surface, which means more grease, smoke, and odors stay in your kitchen. If your hood seems louder than it should be or is not capturing smoke effectively, an undersized duct is one of the first things to check.
Should I use rigid or flexible duct for a range hood?
Rigid metal duct (galvanized steel or aluminum) is always preferred. It has a smooth interior that minimizes air resistance and does not trap grease the way corrugated flex duct does. Flex duct is acceptable only for short transition pieces under 2 feet. For the main run, rigid duct is required by most building codes and recommended by every hood manufacturer.
How do I know what size duct my range hood needs?
Check the installation manual that came with your hood. Every manufacturer lists a minimum duct diameter, and many list a recommended diameter for longer runs. If you do not have the manual, look up the model number on the manufacturer's website. As a general rule, hoods under 600 CFM need at least 6 inches, hoods from 600–900 CFM need 8 inches, and anything above 900 CFM needs 10 inches or larger.
Can I reduce the duct size partway through the run?
You should avoid it. A reducer creates a choke point where pressure builds up and grease accumulates. If space constraints make a full-size run impossible, it is better to switch from round to rectangular duct at the equivalent cross-sectional area than to reduce the diameter. If you must reduce, do it as close to the exterior termination as possible — never immediately at the hood outlet where airflow is highest.
