Range Hood CFM Calculator
This free range hood CFM calculator sizes kitchen exhaust the way the trade does. For an electric or induction cooktop it uses the HVI width rule — 100 CFM per 12 inches of cooktop — and for gas it uses the BTU rule of 1 CFM per 100 BTU of total burner output, taking whichever number is larger. An island hood adds 20 to 30 percent because it has no wall to contain the rising plume, and a downdraft needs more still. All of that sits next to the flat IRC and ASHRAE 62.2 code floor of 100 CFM intermittent (or 25 CFM continuous) so you can see the difference between the legal minimum and a hood that works.
The part that trips people up is makeup air. When a hood is capable of exhausting more than 400 CFM, IRC M1503.4 requires an interlocked makeup-air supply roughly equal to the exhaust rate — without it, a powerful hood can pull a vacuum on a tight house and backdraft a water heater or furnace. The trigger is the hood’s rating, not the speed you run it, and it is strict: 401 CFM triggers the requirement, exactly 400 does not. This calculator tells you which side of that line your hood lands on and how much makeup air it would need.
It also routes you around the common mistakes: a recirculating hood provides no ventilation credit and should be ducted outdoors wherever the layout allows (never into an attic, per IRC M1501.1), the duct should be smooth metal sized to the airflow, and the hood should be at least as wide as the cooktop. No pricing, no signup — just the CFM, the makeup-air answer, and the duct size. It is part of a growing HVAC and ventilation section alongside the bathroom exhaust fan calculator.
Range Hood CFM Calculator
How many CFM does your range hood need? Enter your cooktop and this calculator sizes the hood the right way — by width for electric, by total BTU for gas, plus 25% for an island — shows it against the code minimum, and flags the one thing most tools miss: whether your hood crosses the 400 CFM line that triggers a required makeup-air supply (IRC M1503.4). Free, no signup.
Cooktop
Gas cooktops are sized by total BTU, electric and induction by width. The larger method wins.
Add up every burner’s BTU rating — a typical 4-burner gas range totals 40,000–65,000 BTU; pro ranges reach 90,000+.
Hood & operation
Island hoods pull from all sides (+25%). A downdraft has no capture hood, so it needs more airflow.
Calculation Formulas
IRC M1505.4.4 sets a flat kitchen local-exhaust floor for a vented hood — a low bar that a properly sized hood almost always exceeds.
Example:
Any vented hood must move at least 100 CFM on demand.
The HVI rule of thumb for electric and induction cooktops: 100 CFM for every 12 inches of cooktop width.
Example:
A 30-inch electric cooktop → (30 ÷ 12) × 100 = 250 CFM.
Gas cooktops add combustion heat and byproducts, so HVI sizes them by output: one CFM per 100 BTU. Use the larger of the width and BTU results.
Example:
60,000 BTU total → 60,000 ÷ 100 = 600 CFM.
An island or ceiling-mounted hood is open on all sides with no wall to help contain the plume, so HVI adds 20–30%.
Example:
360 CFM base × 1.25 = 450 CFM for an island.
IRC M1503.4 requires an interlocked makeup-air supply whenever a hood is CAPABLE of exhausting more than 400 CFM. It is the hood’s rating, not the speed you run, that governs — 401 CFM triggers it, exactly 400 does not.
Example:
A 600 CFM hood needs ~600 CFM of makeup air; a 400 CFM hood does not.
An undersized or long, kinked duct chokes a hood as surely as it does a bath fan. Use smooth metal duct and increase one inch for flex.
Example:
A 600 CFM hood wants a 6-inch smooth metal duct.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Kitchen intermittent floor | 100 CFM | IRC M1505.4.4 minimum for a vented hood that runs on demand. |
| Kitchen continuous floor | 25 CFM | Minimum for a hood that runs continuously. |
| Width rate | 100 CFM / 12 in | HVI airflow per foot of cooktop width for electric/induction. |
| BTU rate | 1 CFM / 100 BTU | HVI airflow per unit of gas burner output. |
| Island uplift | +20–30% | HVI adjustment for an island / ceiling-mounted hood. |
| Makeup-air trigger | > 400 CFM | Hood capability above which IRC M1503.4 requires makeup air. |
| Downdraft floor | 300 CFM | ASHRAE 62.2-2019 rate for a downdraft / non-hood system. |
| Hood width | ≥ cooktop | Hood should be at least as wide as the cooktop, ideally 3–6 in. wider. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC M1503 — Range Hoods(IRC M1503)
View StandardGoverns domestic cooking exhaust: hood construction, ducting, and the makeup-air requirement that trips at 400 CFM.
Key Requirements:
- •Exhaust hoods must discharge to the outdoors through a single-wall smooth duct.
- •M1503.4: makeup air required when the hood is capable of exhausting more than 400 CFM.
- •Makeup air must be approximately equal to the exhaust rate, with an automatic or gravity damper.
IRC M1505.4.4 — Kitchen Local Exhaust(IRC M1505.4.4)
View StandardSets the minimum kitchen exhaust airflow — the code floor the hood must at least meet.
Key Requirements:
- •Kitchen: 100 CFM intermittent or 25 CFM continuous.
- •A vented range hood satisfies the requirement; a recirculating hood does not.
ASHRAE 62.2-2019 — Kitchen Ventilation(ASHRAE 62.2)
View StandardThe residential ventilation standard many codes reference; Table 5-1 gives kitchen exhaust demand including the downdraft case.
Key Requirements:
- •Vented hood: 100 CFM intermittent.
- •Downdraft or other (non-hood): 300 CFM.
- •Enclosed kitchen continuous option: 5 air changes per hour.
HVI Kitchen Sizing(HVI)
View StandardThe Home Ventilating Institute publishes the best-practice sizing methods that scale the hood to the cooktop.
Key Requirements:
- •100 CFM per 12 inches of cooktop width (electric/induction).
- •1 CFM per 100 BTU of total burner output (gas).
- •Add 20–30% for an island; hood at least as wide as the cooktop.
IMC §505 — Domestic Kitchen Exhaust(IMC 505)
View StandardThe International Mechanical Code mirror of the range-hood and makeup-air provisions, used where the IMC is adopted instead of the IRC.
Key Requirements:
- •Makeup air for hoods over 400 CFM, matched to the exhaust rate.
- •Ducts to the exterior with backdraft protection.
- •Some jurisdictions (e.g., Minnesota) use a calculated makeup-air method that can trigger below 400 CFM.
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
The 400 CFM Makeup-Air Trigger
Capability, not speed
The IRC/IMC require makeup air when a hood is capable of exhausting more than 400 CFM — the hood’s maximum rating, not the speed you happen to use. A 600 CFM hood run on low still needs makeup air.
Regional Examples:
Gas vs. Electric
Sizing method and combustion
Gas cooktops are sized by BTU and add combustion moisture and byproducts, so they usually call for more airflow than an equal-width electric cooktop. In tight homes a strong gas-range hood is the classic backdraft scenario for water heaters and furnaces.
Regional Examples:
Island & Downdraft
Capture geometry
An island hood has no walls to contain the plume, so HVI adds 20–30%. A downdraft pulls against the rising plume and needs more airflow still, with a 300 CFM floor under ASHRAE 62.2.
Regional Examples:
Ducted vs. Recirculating
Ventilation credit
A recirculating (ductless) hood filters and returns air to the room — it removes some grease and odor but no moisture or combustion byproducts, and provides no code ventilation credit. Duct outdoors whenever the layout allows.
Regional Examples:
California Title 24
Capture efficiency and higher minimums
California adopts ASHRAE 62.2 with amendments and can require a capture-efficiency path (ASTM E3087) or higher airflow set by dwelling floor area and fuel type — with gas ranges required to exhaust more than electric.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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How to Use This Calculator
- Pick your cooktop type — electric/induction (sized by width) or gas (sized by total BTU).
- Select the cooktop width; for gas, also enter the total burner output in BTU (add up every burner).
- Choose the hood mount: wall/under-cabinet, island (+25%), or downdraft.
- Set the operation mode and whether the hood is ducted outdoors or recirculating.
- Click Calculate to get the recommended hood CFM next to the code minimum, and the makeup-air verdict.
- If the makeup-air flag is on, plan an interlocked supply roughly equal to the exhaust rate; note the recommended duct diameter and vent it outdoors.
Why makeup air matters — the 400 CFM rule
IRC M1503.4 requires a makeup-air supply whenever a range hood is capable of exhausting more than 400 CFM. The reason is physics: a strong hood removes a large volume of air, and in a reasonably tight modern home there is nowhere for replacement air to come from except back down the flue of an atmospherically vented water heater or furnace, which can spill combustion gases (including carbon monoxide) into the house. The makeup-air system — an interlocked damper that opens when the hood runs and supplies roughly the same airflow — prevents that. The trigger is the hood’s maximum capability, not the fan speed you use, so a 600 CFM hood run on low still needs makeup air. This calculator sizes the hood and tells you whether you cross the 400 CFM line; it does not design the makeup-air system itself (duct routing, tempering, and interlock are a mechanical-design task), and some jurisdictions such as Minnesota use a calculated method that can require makeup air below 400 CFM, so always confirm your local mechanical code.
Frequently Asked Questions
How many CFM does my range hood need?
Size it to the cooktop, not the code floor. For an electric or induction cooktop, HVI recommends 100 CFM for every 12 inches of width — so 250 CFM for a 30-inch cooktop, 300 for a 36-inch. For gas, use 1 CFM per 100 BTU of total burner output — a 60,000 BTU range wants about 600 CFM — and take whichever number is larger. Add 20 to 30 percent for an island hood. The code minimum is only 100 CFM (IRC M1505.4.4), which most real hoods exceed easily.
Do I need makeup air for my range hood?
You do if the hood is capable of exhausting more than 400 CFM (IRC M1503.4). At that airflow a hood can pull enough of a vacuum on a tight house to backdraft an atmospherically vented water heater or furnace, so the code requires an interlocked makeup-air supply roughly equal to the exhaust rate. The trigger is the hood's rating, not the speed you run it — a 600 CFM hood on low still needs makeup air — and it is exact: 401 CFM triggers it, 400 does not. This calculator tells you which side of the line you're on.
How do I size a range hood for a gas cooktop?
Use the BTU method: 1 CFM per 100 BTU of total burner output. Add up every burner's rating — a typical four-burner gas range totals 40,000 to 65,000 BTU, and pro-style ranges reach 90,000 or more — then divide by 100. A 60,000 BTU range needs about 600 CFM. Compare that to the width method (100 CFM per 12 inches) and use whichever is larger. Because gas adds combustion moisture and byproducts, a captured wall or under-cabinet hood outperforms a downdraft, and the higher airflow often crosses the 400 CFM makeup-air line.
Is a bigger range hood always better?
No — oversizing has real costs. Past 400 CFM you must add a makeup-air system (IRC M1503.4), which adds cost and complexity, and an oversized hood in a tight house can depressurize it and backdraft combustion appliances. It also moves more conditioned air outside than you need and can be noisier. Size to the cooktop with the width or BTU method, add the island adjustment if it applies, and stop there. A right-sized hood ducted with smooth metal and vented outdoors beats an oversized one on flex duct every time.
Does a recirculating range hood count as ventilation?
No. A recirculating (ductless) hood pulls air through a charcoal filter and blows it back into the room. It captures some grease and odor but removes no moisture and no combustion byproducts, and it provides no code ventilation credit — the IRC allows it only where other kitchen ventilation is provided. Whenever the layout allows, duct the hood outdoors through smooth metal duct to an exterior wall or roof cap; it removes far more heat, steam, and pollutants and actually satisfies the kitchen exhaust requirement.
What size duct does a range hood need?
Match the duct to the airflow and keep it smooth metal, short, and straight: 6 inches for hoods up to about 600 CFM, 7 inches to 900, 8 inches to 1200, and 10 inches above that. Increase the diameter by an inch if you must use flex duct, and never reduce below the hood's outlet size. Every elbow and foot of length adds static pressure that strangles delivered airflow, so a big hood on a long, kinked, undersized duct can move less air than a modest hood on a good one. Vent fully outdoors, never into an attic (IRC M1501.1).