How to size a range hood
Start with the hood's width. The Home Ventilating Institute gives its rates per foot of range: 100 CFM per foot recommended against a wall, 40 at the minimum, and 150 and 50 over an island. Those rates are not tied to a fuel. HVI says that for professional-style cooktops you should follow the cooktop maker's advice.
For gas, you may also see a rule of 100 CFM per 10,000 BTU. This comes from a seller, World Coppersmith, and HVI does not state it, so treat it as a rough check. Add up your burners from the manual or the plate on the cooktop. The calculator shows both and looks for the larger one. Compare HVI-certified figures between hoods, since HVI says non-certified hoods often have inflated ratings, and recirculating hoods provide no real ventilation.
Then look at makeup air, as it can change the price of the job. Section M1503.6 of the 2021 and 2024 IRC applies where a system can move over 400 CFM and a fuel-burning appliance that is neither direct-vent nor mechanical-draft sits inside the air barrier. Older editions had no such condition, and neither does South Carolina's text, so check what your area adopted. We read the code through state adoptions on UpCodes, since we could not open ICC's own site.
A worked example
Here is a 30 inch hood on a wall over a gas cooktop with 40,000 BTU across its burners. These lines come from the same formula the calculator above runs.
30 in wall hood, 40,000 BTU
- 30 ÷ 12 = 2.5 ft
- 2.5 × 100 = 250 CFM recommended
- 2.5 × 40 = 100 CFM minimum
- 40000 ÷ 10000 × 100 = 400 CFM on the BTU rule
- Look for about 400 CFM
That sits exactly at 400 CFM, and the code's test is "in excess of" 400, so this hood does not trigger it. Move up to 60,000 BTU, and the rule asks for 600 CFM. If a natural draft appliance is in the home, makeup air then applies at about that same rate, not just the part above 400. A remodel will price this in with the kitchen remodel calculator, and the bathroom fan calculator uses the same institute's method.
Range hood airflow by width
HVI's table prints the 30, 36 and 48 inch wall rows, and the others follow from its rates per foot. The 42 inch row comes to 350 CFM, which is why a seller's 420 CFM for that width does not match HVI's rate.
| Hood | Wall, recommended | Wall, minimum | Island, recommended | Island, minimum |
|---|---|---|---|---|
| 24 in | 200 | 80 | 300 | 100 |
| 30 in | 250 | 100 | 375 | 125 |
| 36 in | 300 | 120 | 450 | 150 |
| 42 in | 350 | 140 | 525 | 175 |
| 48 in | 400 | 160 | 600 | 200 |
| 60 in | 500 | 200 | 750 | 250 |
Mistakes when choosing a hood
- Buying the biggest one. A hood over 400 CFM can bring makeup air into the job, and it may not move more than you need.
- Counting on the low setting. The code tests what a system is capable of exhausting, so a low setting does not avoid it.
- Sizing only the excess. Makeup air under M1503.6 is at about the full exhaust rate. The figure for the excess came from a proposal and is not code.
- Going ductless by default. HVI says nonducted hoods provide no actual ventilation. M1503.3 expects a smooth, airtight duct to the outdoors, with a listed ductless exception.
- Forgetting the damper. M1503.6.2 asks for one that opens when the exhaust runs, by gravity or electrically.
Questions
How many CFM range hood do I need?
About 100 CFM per foot of range against a wall, and 150 CFM per foot over an island, by HVI. That is 250 CFM for a 30 inch wall hood. HVI's minimum rates are 40 and 50 per foot. With gas, check the BTU rule of thumb too.
How many CFM range hood for a gas stove?
About 100 CFM per 10,000 BTU, by a seller's rule of thumb that is not from HVI or a code. A 40,000 BTU cooktop comes to 400 CFM. HVI's rates are not fuel specific, and it says to follow the maker's advice on professional-style cooktops.
How many CFM for a range hood, 30 inch?
250 CFM recommended and 100 CFM minimum against a wall, by HVI's table. Over an island, its rates come to 375 and 125. A gas cooktop with many BTU may call for more on the rule of thumb, and the calculator shows that side by side.
How many CFM per BTU for a range hood?
About 1 CFM for each 100 BTU, as the rule of thumb is 100 CFM per 10,000 BTU. It comes from a seller and not from HVI, which states no rule by BTU. A 100,000 BTU stove comes to 1,000 CFM on this rule, which triggers the makeup air test.
What size range hood do I need for a 36 cooktop?
300 CFM recommended and 120 CFM minimum for a 36 inch hood on a wall, by HVI. Over an island, its rates come to 450 and 150. We have no primary source on how much wider than the cooktop a hood should be, so check the maker's guide.
What are the makeup air requirements for a range hood?
Makeup air at about the exhaust rate if the system can move over 400 CFM and a natural draft fuel appliance sits inside the air barrier, by IRC M1503.6. States differ, as South Carolina has no fuel condition. The test is what it can move, even turned down.
How much does makeup air for a range hood cost?
We have no reliable figure, so get a quote. A system can be passive or mechanical under M1503.6, and M1503.6.2 asks for a damper that opens when the exhaust runs. Ask an HVAC contractor to price the size your hood's capacity calls for.
When do I need a professional?
When a hood's capacity is over 400 CFM in a home with a natural draft furnace, water heater or fireplace, or when you need a new duct through a wall or a roof. An HVAC contractor can size the makeup air, and your inspector decides what applies.
This range hood size is a guide using HVI's rates and 2021 IRC section M1503.6, which states amend. South Carolina's text has no fuel appliance condition at all, so your building department and inspector decide. We read the code through state adoptions, not ICC's own copy. Compare HVI-certified airflow, as uncertified ratings can run high.
Sources and review
- How much ventilation do I need?, Home Ventilating Institute, no date shown, read November 2026.
- IAQ position paper, Home Ventilating Institute, 2009.
- IRC 2021 chapter 15, exhaust systems, International Code Council, via UpCodes, 2021.
- IRC 2024 chapter 15, exhaust systems, International Code Council, via UpCodes, 2024.
- IRC M1503.4 proposal, International Code Council, 2014.
- South Carolina R. 8-1235, Cornell Legal Information Institute, no date shown, read November 2026.
- How much CFM does a range hood need?, World Coppersmith, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
HVI's rate
CFM = Hood width in feet × Rate per foot
- Rate per foot
- 100 recommended on a wall, and 150 over an island
Worked through: 2.5 × 100 = 250
Gas rule of thumb
CFM = Total BTU ÷ 10,000 × 100
- Total BTU
- All burners added up
Worked through: 40000 ÷ 10000 × 100 = 400
The constants, and where they come from
WALL_RECOMMENDED_CFM_PER_FOOT
Published figureHVI's recommended airflow per foot of range, wall hood
Applies to: Hoods in homes. HVI says to follow the maker's advice on professional-style cooktops
Source: How much ventilation do I need?, Home Ventilating Institute, no date on the page, read 2026-11.
Also from: IAQ position paper, Home Ventilating Institute, published 2009-10, read 2026-11.
HVI's width table gives 250 CFM at 30 inches, 300 at 36 and 400 at 48. These rates are not fuel specific.
WALL_MINIMUM_CFM_PER_FOOT
Published figureHVI's minimum airflow per foot of range, wall hood
Applies to: Hoods in homes. HVI says to follow the maker's advice on professional-style cooktops
Source: How much ventilation do I need?, Home Ventilating Institute, no date on the page, read 2026-11.
HVI's width table gives 100 CFM at 30 inches, 120 at 36 and 160 at 48, which match this rate. Its 2009 position paper gives the same 40.
ISLAND_RECOMMENDED_CFM_PER_FOOT
Published figureHVI's recommended airflow per foot of range, island hood
Applies to: Hoods in homes. HVI says to follow the maker's advice on professional-style cooktops
Source: How much ventilation do I need?, Home Ventilating Institute, no date on the page, read 2026-11.
HVI says to multiply the rate by 1.5 for hoods over islands, and its table gives 150. Its 2009 position paper gives the same figure.
ISLAND_MINIMUM_CFM_PER_FOOT
Published range, narrowedHVI's minimum airflow per foot of range, island hood
Applies to: Hoods in homes. HVI says to follow the maker's advice on professional-style cooktops
Source: How much ventilation do I need?, Home Ventilating Institute, no date on the page, read 2026-11.
HVI's table gives 50, though multiplying its wall minimum of 40 by 1.5 would give 60. The table and its sentence disagree here, and we use the table.
CFM_PER_10000_BTU
Modelled from published dataAirflow for each 10,000 BTU of gas cooktop
Applies to: A seller's rule of thumb, which is not from HVI or any code
Source: How much CFM does a range hood need?, World Coppersmith, no date on the page, read 2026-11.
World Coppersmith gives this rule, and says a 100,000 BTU stove needs 1,000 CFM. HVI does not state a rule by BTU. We use the larger of this and HVI's rate.
CODE_MINIMUM_CFM
Published figureIRC minimum for a kitchen fan run intermittently
Applies to: The model IRC, read through state adoptions, which may be amended
Source: IRC 2021 chapter 15, exhaust systems, International Code Council, via UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
Also from: IRC 2024 chapter 15, exhaust systems, International Code Council, via UpCodes, published 2024, read 2026-11.
Table M1505.4.4 in the 2021 IRC and Table M1505.5 in 2024. The code also allows 25 cfm of continuous exhaust, which a hood rarely is.
MAKEUP_AIR_THRESHOLD_CFM
Published figureExhaust above which makeup air may be required
Applies to: The model IRC, read through state adoptions, which may be amended
Source: IRC 2021 chapter 15, exhaust systems, International Code Council, via UpCodes, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
Also from: IRC 2024 chapter 15, exhaust systems, International Code Council, via UpCodes, published 2024, read 2026-11.
Also from: IRC M1503.4 proposal, International Code Council, published 2014-03, read 2026-11.
Section M1503.6 applies only with a fuel-burning appliance that is neither direct-vent nor mechanical-draft inside the air barrier. Older editions and some states have no such condition.
INCHES_PER_FOOT
DefinitionalInches in one foot
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Twelve inches make a foot by definition, so a hood's width in inches is divided by 12 to apply a rate per linear foot.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the range hood CFM calculator
- 1
Choose the hood's width
- 2
Say if it is on a wall or over an island
- 3
Say if the cooktop is gas, and add up its burners
- 4
Say if the home has a natural draft fuel appliance
- 5
Compare HVI-certified ratings, and ask your building department about makeup air
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- The target is the largest of HVI's recommended rate, the BTU rule and the code minimum
- The gas BTU rule is a seller's rule of thumb, not from HVI or any code
- Makeup air is flagged above 400 CFM unless you say there is no such appliance
- Duct length is not counted, since we found no primary figure for it
Pro tips
- Compare HVI-certified CFM between hoods
- Discharge outdoors through a smooth, airtight duct
- Ask your building department which edition it has adopted