Two calculators in one: the residential range hood (cooktop BTU or width, wall vs island, gas vs induction) and the commercial kitchen hood (hood length × code exhaust rate by cooking duty and hood style). Both give the exhaust fan airflow, the make-up air to bring back in, a duct size and the static pressure to expect.
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Hood Exhaust Airflow & Make-Up Air
Residential: 100 CFM per 10,000 BTU/h gas, or 100 CFM per linear foot (wall) / 150 (island). Commercial: CFM per linear foot of hood by duty (IMC Table 507).
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
Residential range hoods — sizing rules
By burner rating (gas) CFM = total BTU/h ÷ 100 // 60,000 BTU/h → 600 CFM By width, wall hood CFM = 100 × cooktop width (ft) // 30 in → 250 CFM, 36 in → 300 CFM By width, island hood CFM = 150 × width (ft) // open on all sides — 50 % more Code minimum 100 CFM intermittent (IRC M1503) / 300 CFM HVI recommendation; make-up air above 400 CFM
Use the larger of the rules. Induction and electric cooktops produce no combustion products, so 250–400 CFM with good capture is plenty. Over-the-range microwaves are limited to 300–400 CFM by their small capture area and recirculation options.
Cooktop
Wall hood
Island hood
Duct
30 in electric / induction
250–300 CFM
400 CFM
6 in
30 in gas, 40k BTU/h
400 CFM
450 CFM
6–7 in
36 in gas, 60k BTU/h
600 CFM
600 CFM
8 in
36 in pro gas, 80k BTU/h
800 CFM
900 CFM
8–10 in
48 in pro gas, 100k+ BTU/h
1,000–1,200 CFM
1,200 CFM
10 in
Commercial hoods — IMC 507 exhaust rates (CFM per linear foot)
Hood type
Light duty
Medium duty
Heavy duty
Extra-heavy
Wall-mounted canopy
200
300
400
550
Single island canopy
400
500
600
700
Double island canopy
250
300
400
550
Backshelf / pass-over
250
300
400
n/a
Eyebrow
250
250
n/a
n/a
Unlisted hoods, IMC 2018/2021 Table 507.5.1. Listed (UL 710) hoods use the manufacturer's tested rate, often 20–40 % lower. Duty: light = ovens, steamers, kettles; medium = ranges, fryers, griddles, tilting skillets; heavy = charbroilers, woks, salamanders, upright broilers; extra-heavy = solid fuel.
Design the whole system: exhaust fan selected at the real static pressure (hood filters 0.3–0.6 in. w.g. + duct), make-up air unit at 80–90 % of exhaust (tempered in cold climates), and the dining room kept slightly positive to the kitchen. Demand-control kitchen ventilation (DCKV) with EC fans cuts exhaust 30–50 % during off-peak hours.
Frequently Asked Questions
How many CFM do I need for a range hood?
For gas: 100 CFM per 10,000 BTU/h of burner rating. For any cooktop: 100 CFM per foot of width for a wall hood, 150 per foot for an island hood, minimum 300 CFM. A typical 36 in gas range needs 600 CFM; a 30 in induction cooktop 250–300 CFM.
Do I need make-up air for my range hood?
In most of the US (IRC M1503.6) and Canada, yes if the hood exceeds 400 CFM: a motorized damper or make-up air unit interlocked with the hood. Without it, a powerful hood depressurises the house and can back-draft gas appliances.
How is a commercial kitchen hood sized?
Exhaust CFM = hood length (ft) × the code rate for the hood type and cooking duty (IMC Table 507.5.1): e.g. a 10 ft wall canopy over charbroilers (heavy duty) needs 10 × 400 = 4,000 CFM. Listed hoods can use their lower tested rate. Make-up air is about 85 % of exhaust.
What duct size for a 600 CFM range hood?
8 in round (or 3.25 × 14 in rectangular), keeping velocity near 1,000 fpm. 6 in is adequate up to about 400 CFM; 10 in for 900–1,200 CFM. Use smooth rigid duct, minimal elbows, and never flexible foil duct for a hood.
Inline or roof-mounted blower vs hood-mounted?
A remote (inline or roof) blower makes the kitchen 10–15 dB quieter and handles higher static pressure; the hood then needs only a light, grease-filtering shell. For commercial Type I hoods a roof upblast fan with the motor out of the airstream is mandatory.
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