What size range hood do you need over a 30 inch range
A hood is bought by its airflow and installed by its duct. One hood manufacturer counts a 90 degree elbow as 15 equivalent feet, a 45 degree elbow as 9, and a wall cap with a damper as 30, against a total that the same page says should not exceed 150 equivalent feet for its own hoods.
Updated 2026-09-24. General planning information, not professional advice. Every figure on this page names the document it came from.
Airflow is chosen first, and it is the easy part
Airflow is the number on the box, measured in cubic feet per minute, and it is the number everybody shops on. Two published figures bracket it, and they are different kinds of number, so they are worth keeping apart.
Published planning guideline. Published planning guidelines recommend a minimum of 150 cfm for ventilation over a cooking surface. That is a recommendation from an industry body, not a requirement, and nobody inspects it.
Model code figure, verify locally. A minimum exhaust rate of 100 cfm for a ducted hood, ducted to the outside, is the figure in the model code text quoted by the planning guidelines, and it is required in many US jurisdictions. What applies where you live is decided by your building department, not by this page.
Beyond those two, the airflow that suits a particular hood over a particular cooking appliance is a question for the hood manufacturer document and for the person installing it. Some hoods are not a single figure at all: one manufacturer publishes three selectable maximum fan speeds, 590, 390 or 290 cfm, for one of its own island hoods, and states that once the jumper has been repositioned and the hood powered on, the cfm cannot be changed again. That is one product, from its own manual, and it is a good reminder to read yours before the installer leaves.
What this page will not give you is a rule connecting hood width to range width. That relationship was looked for in citable published sources during the build and could not be confirmed, so it is not printed here. Ask the manufacturer of the hood you are considering what width it publishes for the cooking surface below it.
Why the duct route matters more than the rating on the box
A hood moves the air it is rated for through a duct that resists it. Every bend, every cap and every reduction in that duct costs airflow, and the cost is not small. Manufacturers publish the cost as equivalent feet: a fitting is expressed as the length of straight duct that would resist the air by the same amount.
Manufacturer document. The equivalent feet table below is published by one manufacturer, in the installation manual for one of its own island hoods, for use with that hood. Makers count differently and publish different tables. The table in the manual that comes in your own box is the one that governs your installation.
| The fitting | What it counts as | Note |
|---|---|---|
| Round straight duct | 1 equivalent foot per foot | Published for 6, 7, 8 and 10 inch round duct |
| Round 90 degree elbow | 15 equivalent feet | One bend costs fifteen times its own length |
| Round 45 degree elbow | 9 equivalent feet | Two 45 degree bends cost more than one 90 |
| Wall cap with a damper | 30 equivalent feet | The termination, counted once |
| Roof cap | 30 equivalent feet | Same figure as the wall cap with a damper |
| Tapered reducer | 25 equivalent feet | Published for a 7 inch to 6 inch or an 8 inch to 7 inch reduction |
| The total for the whole run | Should not exceed 150 equivalent feet | Stated on the same page as the figures above |
Every figure in this table is published by one manufacturer in the installation manual for one of its own island hoods, and applies to that hood. It is not a general rule and it does not transfer to another maker. Check the table in your own manual.
Equivalent feet are printed here in feet because that is how the manufacturer prints them. They are not a distance you can measure with a tape, they are a resistance expressed as a distance.
How to count your own run in equivalent feet
Take a duct route that any installer would describe as short. The hood is on an interior wall, the duct goes up into the joist space, turns to run out to the exterior wall, turns again to go through it, and ends in a wall cap with a damper.
Counted with the figures that one manufacturer publishes for its own hoods, that route contains two 90 degree elbows at 15 equivalent feet each and one wall cap with a damper at 30. That is 60 equivalent feet before a single foot of straight duct has been counted, against a total the same page says should not exceed 150.
A physically short run with two bends and a cap is already 40 percent of the way to the limit that one manufacturer publishes for its own hoods, before any straight duct is added.
Add 14 feet of straight duct to that example and the same arithmetic gives 74 equivalent feet. Add a third elbow to get around a beam and it is 89. Reduce from 8 inch to 7 inch to fit through a joist and, on this manufacturer table, the tapered reducer adds 25 more. The route is now 114 equivalent feet and nobody has drawn anything unusual.
The numbers above are an example worked with one manufacturer published table. They describe the example and nothing else. What matters is that the arithmetic exists and that somebody does it for your route, using your own manual, before the ceiling closes. That somebody is your installer, and the question is in the list at the end of this page.
Duct diameter, and what flexible duct costs you
Diameter is published per hood, and the published figures vary a great deal between products, which is exactly why a general rule is not printed here.
- One manufacturer recommends a minimum of 8 inch (203 mm) round duct for one of its own island hoods to maintain airflow, and says to increase from 8 inch to 10 inch (254 mm) where a long duct run is required.
- A different manufacturer specifies 6 inch (152 mm) round duct for one of its own wall hoods, and states that only metal ductwork is to be used.
Those are two named products from two published documents, and they disagree by two inches of diameter because they are different hoods. Neither figure is a rule about range hoods, and taking either one to your own installation is how a duct gets built to the wrong size.
Manufacturer document. One manufacturer states in the manual for its own island hood that flexible ducts could restrict air flow by up to 50 percent. That is a figure that manufacturer publishes about flexible duct used with its own hoods. It is the single most useful sentence in that document and it is the reason to ask, in advance, whether any part of your run is flexible and how long that part is.
The second manufacturer document goes further in a different direction, saying plainly that long duct runs, elbows and transitions will reduce the performance of the hood, and to use as few of them as possible. Again, that is that maker, writing about its own hood. The consistency between the two is a hint, not a citation.
How high the hood goes, and why four documents disagree
The distance between the cooking surface and the bottom of the hood is the one measurement in a kitchen where a planning guideline, a model code figure and two manufacturer manuals all give a number, and all four numbers are different.
| Who publishes it | The distance above the cooking surface | What kind of number it is |
|---|---|---|
| Published planning guidelines | 24 in (610 mm) | A recommendation, to a protected noncombustible surface above the cooking surface. Not code |
| Model code text quoted by the guidelines | 30 in (762 mm), required in many US jurisdictions, verify locally, with 24 in (610 mm) permitted where an approved hood or other approved noncombustible material is installed | A code figure to combustible material and metal cabinets, required in many US jurisdictions, verify locally with your building department |
| One wall hood manual | Not less than 30 in (762 mm), with 36 in (914 mm) given as the maximum recommended for capture | One manufacturer figure for one of its own wall hoods |
| One island hood manual | Not less than 24 in (610 mm), and no higher than 34 in (864 mm) | One manufacturer figure for one of its own island hoods |
Four documents, one distance. Rows one and two come from published planning guidelines, row two quoting model code text that is required in many US jurisdictions and that your building department decides on. Rows three and four are two named manufacturers writing about their own hoods.
Where those figures differ, your installer and your building department decide which distance governs, and the manual for the hood you actually bought is one of the documents they work from. Ask which one applies before the hood is ordered, because the answer moves the wall cabinets above the cooking surface and sometimes the soffit.
When is make-up air required for a range hood
Two figures circulate for when make-up air is needed, and they are not the same kind of number.
| The figure | What it actually is | Who decides |
|---|---|---|
| 300 cfm | The airflow above which one manufacturer publishes a make-up air kit for use with any of its own hoods. A motorized damper that opens when the hood runs and closes when it stops | That manufacturer, about its own products |
| 400 cfm, required in many US jurisdictions, verify locally | The figure in the model code text quoted by the planning guidelines: an exhaust hood system capable of exhausting in excess of 400 cfm is to be provided with make-up air. Required in many US jurisdictions, verify locally | Your building department, through your contractor |
The 300 cfm figure is where one maker begins selling a kit for its own hoods. The 400 cfm figure is code text quoted by published planning guidelines, required in many US jurisdictions, and what applies where you live is decided by your building department. The two numbers do not mean the same thing.
The same manufacturer sizes its own damper by the airflow of the hood: 6 inch (152 mm) round for a hood up to 600 cfm, 8 inch (203 mm) up to 1000 cfm, and 10 inch (254 mm) up to 1300 cfm. Those are that product line, from that published spec sheet.
Model code figure, verify locally. Whether a make-up air provision is required for your kitchen, and above what airflow, is required in many US jurisdictions and is decided where you live, by your building department, through your contractor. Ask before the hood is ordered, because the answer changes what opening has to be cut in the wall.
That timing is the point. Make-up air is not an accessory bought afterwards. It is a hole in the building envelope, a damper and a control wire, and all three belong to the days when the walls are still open.
The three questions to ask on the day the duct is run
- Where does this duct come out of the building, and who has stood outside and seen that point.
- How many bends are in the run, what diameter is it for its whole length, and what does the route come to in equivalent feet on the table in this hood manual.
- Is any part of the run flexible, and if so, how long is that part.
None of the three is a question you answer yourself, and none of them can be answered after the ceiling is closed. Ask them on the day the duct is run, get the answer from the installer, and photograph the run with the bends visible.
Nothing on this page asks you to install, modify, test or inspect a duct, a damper or an electrical connection. Ductwork, gas and wiring are the work of the licensed trades who carry them, and the only job described here is looking, photographing and asking while looking is still possible.
Common questions
How many cfm do I need for a range hood?
Published planning guidelines recommend a minimum of 150 cfm for ventilation over a cooking surface, and the model code text those guidelines quote sets a minimum exhaust rate of 100 cfm for a ducted hood, ducted to the outside, required in many US jurisdictions and decided where you live by your building department. Beyond those figures, the airflow for a particular hood is published by its manufacturer.
What is equivalent duct length for a range hood?
Equivalent duct length expresses each fitting in a duct run as the length of straight duct that would resist airflow by the same amount. One hood manufacturer publishes 15 equivalent feet for a 90 degree elbow, 9 for a 45 degree elbow and 30 for a wall cap with a damper, against a total that should not exceed 150 equivalent feet for its own hoods.
How much airflow does a 90 degree elbow cost?
One hood manufacturer counts a round 90 degree elbow as 15 equivalent feet in the installation manual for one of its own island hoods, against a total the same page says should not exceed 150 equivalent feet. Makers count differently, so the table in your own hood manual is the one that governs your run.
Should a range hood duct be 8 inch or 10 inch?
It is published per hood, not as a rule. One manufacturer recommends a minimum of 8 inch (203 mm) round duct for one of its own island hoods and says to increase to 10 inch (254 mm) where a long run is required. A different manufacturer specifies 6 inch (152 mm) round duct, metal only, for one of its own wall hoods.
Does flexible duct reduce range hood airflow?
One manufacturer states in the installation manual for one of its own island hoods that flexible ducts could restrict air flow by up to 50 percent. That is one maker publishing a figure about flexible duct used with its own hoods, and it is a reason to ask in advance whether any part of a run is flexible and how long that part is.
When is make-up air required for a range hood?
Two different figures circulate. 300 cfm is where one manufacturer begins publishing a make-up air kit for its own hoods. 400 cfm is the figure in the model code text quoted by published planning guidelines, above which an exhaust hood system is to be provided with make-up air, required in many US jurisdictions. What applies where you live is decided by your building department.