Range Hood CFM Calculator

Size kitchen exhaust with this range hood CFM calculator: cooktop width and burner BTU set the airflow, then the duct run and the 400 CFM makeup air rule set the rest.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: Take the larger of 100 CFM per foot of cooktop width, 150 for an island, and 1 CFM per 100 BTU of gas burner output, then check the duct. A 30 in gas cooktop rated 40,000 BTU under a wall hood needs 400 CFM, the larger of 250 CFM by width and 400 CFM by burner output.

Your measurements

A rectangular volume: length times width times depth.

Cooktop typeGas adds combustion products to the plume, so the burner rule applies on top of the width rule. Pro-style gas ranges are rated so hot that makers typically ask for at least 600 CFM.Cooktop widthinThe cooking surface, not the hood. The hood itself should match the cooktop on a wall and run 3 in past each side on an island.Total burner output (BTU per hour)Add the burner ratings from the range's rating plate. A 30 in gas cooktop totals 35,000 to 45,000 BTU and a 36 in pro range 60,000 to 90,000. Ignored for electric and induction.Hood styleAn island hood has no walls to hold the plume, so the rule goes to 150 CFM per foot. A downdraft fights the rising plume the whole way and still captures the back burners better than the front.Straight duct runftMeasured duct only. Elbows and the exterior cap are added separately as equivalent length.90 degree elbows in the runCount the turn out of the hood and the turn at the wall. Each one costs about as much pressure as 15 ft of straight duct. Two 45s in a row count as one 90.Duct diameterThe duct already in the wall, or the one you plan to run. A 3-1/4 x 10 in rectangular duct behaves like 6 in round. Transitions down to the hood collar do not count as the duct size.

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Results

Exhaust airflow needed: 400 CFM; The larger of 250 CFM from the under-cabinet width rule and 400 CFM from the burner output rule
Recommended duct diameter: 8 in round; Smallest smooth metal size that carries 400 CFM over 65 ft of equivalent length and stays under 1,200 FPM
Effective airflow through your duct: 318 CFM; 6 in round smooth metal at 1,619 FPM, working against 65 ft of equivalent length. Warning: 82 CFM short of what the hood needs, so go up to 8 in or shorten the run
Duct equivalent length: 65 ft; 10 ft of straight duct, 2 elbows at 15 ft each, and 25 ft for the cap and its damper
Makeup air: Not required; IRC M1503.6 triggers above 400 CFM. Below it, still give the kitchen a path for replacement air or the hood will not reach its rating
Sound rating to look for: 5 sones or less on high; HVI rates hoods in sones at 0.1 in w.c., and a hood at 8 sones gets switched off. Moving the blower into the attic or onto the wall drops the kitchen to 2 or 3 sones for typically $300 to $900 more

Range Hood CFM formula

Width rule (CFM) = Cooktop width (in) / 12 x 100 for a wall hood, x 150 for an island
Burner rule (CFM) = Total burner BTU per hour / 100   (gas only)
CFM needed = the larger of the two rules;  pro-style gas ranges start at 600 CFM
Equivalent length = Straight duct + 15 ft per 90 elbow + 25 ft for the cap

Worked example

A 36 in pro gas range rated 60,000 BTU under an island hood, ducted 16 ft with three elbows in 8 in round. The width rule gives 36 / 12 x 150 = 450 CFM. The burner rule gives 60,000 / 100 = 600 CFM. The larger is 600 CFM, which also matches the pro-range minimum.

Equivalent length is 16 + 3 x 15 + 25 = 86 ft, so the blower's 0.5 in w.c. spends 0.58 in w.c. per 100 ft. An 8 in duct passes about 587 CFM at that rate, just short of the 600 needed. At 600 CFM an 8 in duct also runs 1,719 feet per minute, well past the 1,200 FPM noise ceiling. Going to 10 in round drops it to 1,100 FPM. At 600 CFM the hood is over the 400 CFM line, so IRC M1503.6 requires makeup air at roughly 600 CFM.

Quick reference

Range hood CFM by cooktop width and burner output
Cooktop width	Wall hood at 100 CFM per ft	Island hood at 150 CFM per ft	Gas at 1 CFM per 100 BTU
24 in	200 CFM	300 CFM	30,000 BTU = 300 CFM
30 in	250 CFM	375 CFM	40,000 BTU = 400 CFM
36 in	300 CFM	450 CFM	60,000 BTU = 600 CFM
42 in	350 CFM	525 CFM	70,000 BTU = 700 CFM
48 in	400 CFM	600 CFM	90,000 BTU = 900 CFM
60 in	500 CFM	750 CFM	120,000 BTU = 1,200 CFM

Two rules, and you use the bigger one

Kitchen exhaust is sized two ways and the larger answer wins. The width rule is 100 CFM for every foot of cooktop, or 150 for an island, because an island hood has no walls to hold the plume in place. The burner rule applies to gas: 1 CFM for every 100 BTU per hour of total burner output. A 30 in gas cooktop is 2.5 ft wide, so the width rule asks for 250 CFM, but 40,000 BTU of burners asks for 400. Build to 400.

Capture beats raw airflow. A hood that covers the cooking surface at its listed mounting height catches more at 400 CFM than a taller, narrower hood at 600. Wall hoods list 24 to 30 in above the burners. Over an island go 3 in wider than the cooktop on each side. Downdrafts are the weakest arrangement because they pull against a plume that wants to rise. They lose the front burners first, where a tall pot blocks the path. Induction changes the math: there is no flame and no combustion product, so the width rule alone governs and a quieter 300 to 400 CFM hood does the job.

The duct, and the 400 CFM makeup air line

The CFM on the box is measured by HVI at 0.1 inches of water column. Your duct run costs more than that. Count equivalent length, not measured length: roughly 15 ft for each 90 degree elbow and 25 to 40 ft for the exterior cap and its damper. A 10 ft run with two elbows is already about 65 ft, and a 400 CFM hood on 6 in round over that distance delivers around 318 CFM. Size the duct so the hood reaches its rating and keep velocity under about 1,200 feet per minute, which is where the duct itself starts to be the noise. Smooth galvanized, stainless or copper only (IRC M1503.3), joints taped or sealed, and the run pitched slightly toward the outside so grease drains out.

Past 400 CFM the code changes. IRC M1503.6 requires makeup air at approximately the exhaust rate for any hood system capable of exhausting more than 400 cfm. The reason is combustion safety. A tight house cannot replace 600 CFM through its leaks, so the easiest path for replacement air becomes the water heater flue or the fireplace. Makeup air kits run a damper interlocked with the hood, and a tempered kit that pre-heats the incoming air typically costs $800 to $2,500 installed. Plan the wall penetration during framing; retrofitting one through finished drywall and siding is the expensive version.

What this calculator leaves out

This gives airflow and duct size, not a kitchen ventilation design. It does not size the makeup air duct, damper or tempering coil, or check the combustion appliance zone for depressurization, which is a blower door and a manometer job. Not included: the hood itself, typically $200 to $600 for an under-cabinet unit and $700 to $3,000 for a chimney or island hood. Nor an inline or exterior blower, the roof or wall cap and damper, or the duct and fittings. Nor framing and the soffit or chase that hides the run, a dedicated circuit where the nameplate calls for one, or the mechanical permit at typically $50 to $250. Filters, lighting and the makeup air control also sit outside the number here.

Code limits that apply

IRC M1503.6checked
A hood able to exhaust more than 400 cfm needs makeup air at about the exhaust rate
If you miss it: The house goes negative, an atmospheric water heater or fireplace backdrafts, and flue gas enters the kitchen
IRC M1503.4
Range hood exhaust discharges to the outdoors, never into an attic, crawlspace or soffit
If you miss it: Grease and moisture soak the sheathing and insulation, and the inspector fails the rough-in
IRC M1503.3
Duct of galvanized steel, stainless steel or copper, smooth interior, airtight, with a damper
If you miss it: Corrugated flex traps grease into a fire load and strangles the airflow the hood was bought for
IRC M1505.4
Kitchen local exhaust of at least 100 cfm intermittent, vented outdoors
If you miss it: A recirculating hood alone does not satisfy local exhaust and the final inspection stalls
IRC M1503.1
Hoods are installed per their listing, including the mounting height over the cooking surface
If you miss it: A hood hung above its listed height loses capture and voids the listing the inspector checks

Provision	Limit	If you miss it
IRC M1503.6checked	A hood able to exhaust more than 400 cfm needs makeup air at about the exhaust rate	The house goes negative, an atmospheric water heater or fireplace backdrafts, and flue gas enters the kitchen
IRC M1503.4	Range hood exhaust discharges to the outdoors, never into an attic, crawlspace or soffit	Grease and moisture soak the sheathing and insulation, and the inspector fails the rough-in
IRC M1503.3	Duct of galvanized steel, stainless steel or copper, smooth interior, airtight, with a damper	Corrugated flex traps grease into a fire load and strangles the airflow the hood was bought for
IRC M1505.4	Kitchen local exhaust of at least 100 cfm intermittent, vented outdoors	A recirculating hood alone does not satisfy local exhaust and the final inspection stalls
IRC M1503.1	Hoods are installed per their listing, including the mounting height over the cooking surface	A hood hung above its listed height loses capture and voids the listing the inspector checks

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

A 400 CFM hood on 6 in duct with two elbows passes about 318 CFM; the label is measured at 0.1 in w.c.
Over-the-range microwaves ship set to recirculate; the blower has to be rotated and the top or rear knockout opened to vent outside.
A 900 CFM hood with no makeup air pulls the house negative and spills flue gas from an atmospheric water heater.
Corrugated dryer-style flex on a hood collects grease and cuts airflow; IRC M1503.3 wants smooth galvanized, stainless or copper.
Hung 36 in above the burners for headroom, a wall hood stops catching the front burners; makers list 24 to 30 in.
An island hood only as wide as the cooktop misses the edges; go 3 in past each side.

Key facts

IRC M1503.6 requires makeup air for any range hood system capable of exhausting more than 400 cfm, supplied at approximately the exhaust rate.
The linear rule for kitchen exhaust is 100 CFM per foot of cooktop width for a wall hood and 150 CFM per foot for an island hood.
The gas rule is 1 CFM for every 100 BTU per hour of total burner output, so a 60,000 BTU range calls for 600 CFM.
IRC M1505.4 sets the kitchen local exhaust minimum at 100 cfm intermittent, which is less than a third of what a 30 in gas range actually needs.
Hood manufacturers count a 90 degree elbow as roughly 15 feet of equivalent duct and a wall or roof cap as 25 to 40 feet. A 10 ft run with two elbows is really about 65 ft.

Frequently asked questions

How many CFM do I need for a range hood?

Take the larger of 100 CFM per foot of cooktop width (150 for an island) and, for gas, 1 CFM per 100 BTU of total burner output. A 30 in gas cooktop rated 40,000 BTU needs 400 CFM. A 30 in electric or induction cooktop needs only 250 CFM.

Does a range hood over 400 CFM need makeup air?

Yes. IRC M1503.6 requires makeup air at approximately the exhaust rate for any hood system capable of exhausting more than 400 cfm. Without it a tight house goes negative and can pull flue gas back down a water heater or fireplace chimney.

What size duct do I need for a 600 CFM range hood?

Usually 8 in round on a short run and 10 in on a long one. At 600 CFM an 8 in duct runs about 1,719 feet per minute, past the 1,200 FPM point where the duct gets loud. A 10 in duct drops it to 1,100 FPM.

Is a 400 CFM hood enough for a gas range?

For a 30 in range totaling about 40,000 BTU, yes, and it stays under the makeup air threshold. A 36 in range at 60,000 BTU needs 600 CFM and triggers IRC M1503.6. Check the rating plate rather than the range's width.

Sources and references

Chapter 15 Exhaust Systems, Section M1503 range hoods and M1503.6 makeup air, 2021 IRC (ICC)
Kitchen Exhaust Fans: capture efficiency, sizing and duct guidance (U.S. Department of Energy, Building America Solution Center)
HVI Certified Products Directory: certified airflow and sone ratings (Home Ventilating Institute)
HVI publications on home ventilating product ratings and test standards (Home Ventilating Institute)

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