What Size Duct Do I Need for a Range Hood? A Simple Guide to Getting It Right

If you’ve ever asked, what size duct do I need for a range hood, you’re probably standing in the middle of a kitchen renovation wondering why choosing a pipe suddenly feels harder than choosing the beautiful hood hanging above the stove.

You found the hood.

You picked the finish.

You imagined how good it would look over the range.

Then someone asks, “What size is the duct?”

And suddenly you’re looking at numbers like 6, 7, 8, and 10 inches, wondering whether you accidentally signed up for an engineering degree.

Don’t worry. I didn’t sign up either, but we will answer this together.

Range hood ductwork is much easier to understand once you know what you’re looking at.

The most important rule is simple:

The hood manufacturer’s required duct size comes first.

There isn’t one magic duct size that works for every range hood.

A small residential hood and a powerful high-CFM hood can have very different duct requirements.

And choosing the duct first, then hoping your hood likes it, is putting the cart very firmly in front of the horse.

What Is Range Hood Ductwork?

The duct is simply the pathway that carries air from your range hood to the outside.

Your hood captures cooking air.

The duct gives that air somewhere to go.

However, the pathway matters.

Imagine trying to empty a bathtub through a drinking straw.

Technically, the water has somewhere to go.

But you’re going to be waiting a while for that to happen.

A range hood works in much the same way.

If the duct is too small or too restrictive for the hood, airflow can suffer greatly.

That can mean weaker smoke capture, more noise, and a ventilation system that isn’t giving you what you expected.

What Size Duct Do I Need for a Range Hood?

So, what size duct do I need for a range hood?

Start with the installation manual for your specific hood.

That is the safest answer because manufacturers design their blowers and ventilation systems around particular duct requirements.

Broan-NuTone, for example, says many of its residential range hoods use a minimum duct size equivalent to a 6-inch round duct, while more powerful models can require larger ductwork.

That means you shouldn’t look at a hood and say, “Six inches seems normal, so I’ll use six inches.”

You need to look at the hood.

The hood gets the final vote.

Why Range Hood Duct Size Matters

Your range hood duct size affects how easily air can travel from the hood to the outdoors.

When the duct is appropriately sized, the blower has a better path for moving air.

When the duct is too restrictive, the blower has to work against greater resistance.

That can affect airflow.

It can also contribute to more noise.

The goal is simple:

Give the hood a duct that matches what it was designed to use.

What Is a 6-Inch Range Hood Duct Used For?

A 6-inch range hood duct is common on many residential range hoods.

You’ll often see it on smaller or lower-airflow models.

But don’t take that to mean every hood under a certain CFM automatically needs six inches.

Don’t join the many people who got in trouble thinking this. 

CFM and duct diameter are related, but there isn’t one universal formula that says every hood with a particular CFM must use one exact duct size.

Manufacturers specify duct requirements based on the design of the hood and blower.

So if your hood says it needs a 6-inch round duct, that’s what you should plan around.

If it says 8 inches, don’t squeeze it into six because the existing hole in the wall happens to be six inches.

Your kitchen renovation may forgive many things.

Airflow physics is less forgiving.

What Is a 7-Inch Range Hood Duct?

A 7-inch range hood duct is another size you’ll encounter with residential ventilation.

Some models are designed for 7-inch round ducting. This is a good reminder that two range hoods can look almost identical from the front while requiring completely different ductwork behind the scenes.

That’s why product specifications matter.

The pretty stainless-steel box is only half the story.

The invisible pipe is an important part of this equation.

When Do You Need an 8 Inch Range Hood Duct?

An 8-inch range hood duct is commonly found on higher-airflow systems, but again, don’t use the duct size as a shortcut for choosing a hood.

Check the manufacturer’s instructions.

If the hood requires an 8-inch duct, use an 8-inch duct.

If it specifies another size, follow that specification.

And if you’re planning a high-CFM hood before the kitchen is built, this is the time to think about ductwork.

Not after your cabinets are installed.

Not after the ceiling is closed.

And definitely not after the contractor says, “Oh, we already ran the duct.”

That’s a sentence you want to hear before installation, not after.

Can You Use a Smaller Duct Than the Hood Requires?

This is one of those tempting shortcuts that sounds harmless.

The answer is generally no.

If the hood manufacturer specifies an 8-inch duct, reducing it to 6 inches simply because that’s what you have available can restrict airflow.

Like I always say. Never reduce the duct size below the size specified for the hood because doing so can restrict airflow.

Think about it this way.

The hood is designed to move a certain amount of air.

You don’t want to make the air squeeze through a smaller doorway on its way out.

That defeats the whole point.

Does Duct Size Affect CFM?

Yes, but not in the way people sometimes assume.

The CFM rating tells you how much air the hood is capable of moving under specified conditions.

The duct provides the path that air has to travel through.

If the duct is restrictive, the hood may not deliver the same airflow you expected once everything is installed.

That’s why you can’t look at a product page, see “1,000 CFM” and assume you’re automatically getting 1,000 CFM at the cooking surface.

The ventilation system has to be able to support that airflow.

This is really important with high-powered range hoods.

More airflow means you need to pay closer attention to how that airflow gets outside.

How Long Can a Range Hood Duct Be?

Here’s where the duct conversation gets even more interesting.

A duct doesn’t have to be extremely short to work.

But shorter and straighter is generally better.

Most brands recommend keeping the duct run as short and straight as practical and minimizing turns.

Why?

Because every foot and every change in direction can add resistance.

Think of walking from your kitchen to the front door.

If it’s ten straight steps, easy.

If someone makes you walk ten steps, turn left, walk six steps, turn right, walk twelve steps, turn around and then crawl through a cupboard…

Well, you get the point.

You’ll definitely take longer to get there.

Air has its own version of this frustration.

How Many Bends Can a Range Hood Duct Have?

There isn’t one universal number that applies to every range hood.

That’s why range hood duct bends need to be considered as part of the entire duct run.

Every bend can add resistance.

A long duct with multiple sharp turns can be more restrictive than a shorter, straighter duct.

Where bends are unavoidable, smoother turns are generally preferable.

Use the shortest route with the fewest turns possible. 45-degree turns are preferable to 90-degree turns where practical.

So if you’re designing the duct route from scratch, don’t create a maze just because you can.

Your hood doesn’t need an adventure.

It needs an exit.

Is Flexible Ducting Good for a Range Hood?

Flexible ducting might seem convenient because you can bend it around obstacles.

And that’s precisely the problem.

A range hood needs a good airflow path.

Flexible duct can introduce additional resistance and turbulence, especially when it is compressed, bent, or poorly supported.

Always use metal ducting for range hood ventilation. 

Never, and I mean NEVER, use plastic ducting.

Rigid metal ductwork is usually the better choice when you’re installing a permanent range hood ventilation system.

Yes, it can be more work.

But this is one of those situations where “easy now” can become “why is my range hood so loud?” later.

Can You Use a Dryer Vent for a Range Hood?

Please don’t look at that little dryer duct and think, “It’s a tube. Close enough.”

Range hood ventilation has different requirements.

Also, never use 4-inch dryer duct for range hood ventilation.

A range hood needs ductwork sized and configured for the appliance.

Your dryer already has enough drama with lint.

Let it keep its duct.

Does a Longer Duct Make a Range Hood Weaker?

It can.

The longer the path, the more opportunity there is for airflow resistance.

This doesn’t mean your hood has to be directly beside an exterior wall.

It means the duct route should be planned thoughtfully.

A long, smooth, appropriately sized duct can be a very different situation from a long duct filled with bends and restrictions.

That’s why the complete installation matters more than simply counting feet.

What Happens If the Duct Is Too Small?

If your duct is too small for the hood, you may notice several things.

The hood may seem weaker.

Smoke may not disappear as quickly.

The fan may sound louder.

And the system may not move air as efficiently as you expected.

You may also find yourself turning the hood to maximum speed every time you cook.

And if maximum speed still isn’t enough?

That’s when the ductwork deserves serious attention.

A bigger blower can’t magically make an undersized duct disappear.

Does a Bigger Duct Always Mean Better Ventilation?

Not necessarily.

This is another common misconception.

You don’t want to randomly install the biggest duct you can find and assume your hood will suddenly become a ventilation superhero.

The duct should be appropriate for the hood.

Too small can create restrictions.

But the solution isn’t to ignore the manufacturer’s specifications and install something enormous.

The hood, blower, duct diameter, and overall ventilation route should be treated as one system.