# Mechanical Exhaust Requirements: Bathroom, Kitchen and Dryer Exhaust (IRC and IMC)

> The IRC and IMC require bathroom, kitchen and dryer exhaust to discharge outdoors at minimum airflow rates, limit 4-inch dryer ducts to 35 feet less fitting allowances, and set termination clearances from property lines, openings and air intakes.

- Canonical: https://harth.build/resources/design-standards/mechanical-exhaust-requirements/
- Last updated: 2026-10-27
- Data source: Dwelling requirements are taken from [2021 International Residential Code Chapter 15, Exhaust Systems](https://codes.iccsafe.org/content/IRC2021P2/chapter-15-exhaust-systems) (Sections M1501 to M1505 and Tables M1502.4.6.1, M1504.2 and M1505.4.4) and [Chapter 3](https://codes.iccsafe.org/content/IRC2021P2/chapter-3-building-planning) (Section R303.3). Commercial and multifamily requirements come from the [2021 International Mechanical Code, Chapter 5, Exhaust Systems](https://codes.iccsafe.org/content/IMC2021P2/chapter-5-exhaust-systems) (Sections 501, 504 and 505) and [Chapter 4, Ventilation](https://codes.iccsafe.org/content/IMC2021P2/chapter-4-ventilation) (Table 403.3.1.1). The duct-length worked examples are calculated by Harth. These are typical model-code values; verify every rate, length and clearance against the edition adopted by the authority having jurisdiction, the applicable energy code and local amendments.

## Key facts

| Fact | Value |
| --- | --- |
| Bathroom exhaust (house) | 50 cfm intermittent or 20 cfm continuous (IRC 2021 Table M1505.4.4; rated at 0.25 inch water column; required where the room lacks 3 sq ft of glazing (R303.3)) |
| Kitchen exhaust (house) | 100 cfm intermittent or 25 cfm continuous (IRC Table M1505.4.4; IMC Table 403.3.1.1 lists 100 intermittent or 50 continuous for dwelling kitchens) |
| Dryer duct length | 35 feet, less fitting allowances (IRC M1502.4.6.1 and IMC 504.9.4.1; a 4-inch mitered 90-degree elbow counts as 5 feet) |
| Dryer duct | 4-inch smooth metal, transition duct 8 feet maximum (IRC M1502.4.1 and M1502.4.3; no screens at the termination) |
| Range hood makeup air | Required above 400 cfm (IMC 505.4; IRC M1503.6 applies where a non-direct-vent fuel-burning appliance is inside the air barrier) |
| Public toilet room exhaust | 70 cfm intermittent or 50 cfm continuous per fixture (IMC 2021 Table 403.3.1.1, per water closet or urinal) |

## Direct answer

Mechanical exhaust for dwellings is governed by IRC Chapter 15, and for other buildings by IMC Chapters 4 and 5. A dwelling bathroom without 3 square feet of glazing, half of it openable, needs local exhaust of 50 cfm intermittent or 20 cfm continuous under IRC Section R303.3 and Table M1505.4.4, and a ducted kitchen hood must move 100 cfm intermittent or 25 cfm continuous. All exhaust discharges outdoors, never into an attic, soffit, ridge vent or crawl space. Clothes dryer exhaust uses a 4-inch smooth metal duct limited to 35 feet from the transition duct to the outlet, minus 5 feet for each 4-inch radius mitered 90-degree elbow, with a backdraft damper, no screen, and a termination at least 3 feet from building openings. Range hoods over 400 cfm need makeup air under IMC Section 505.4. IMC Table 403.3.1.1 sets commercial exhaust rates, such as 70 cfm intermittent per public water closet or urinal and 0.75 cfm per square foot for parking garages.

*Content and cited inputs reviewed by [Harth team](https://www.linkedin.com/company/harth-build) · Published 2026-10-27 · Updated 2026-10-27.*

## What are the key takeaways?

- Bathroom exhaust (house): 50 cfm intermittent or 20 cfm continuous
- Kitchen exhaust (house): 100 cfm intermittent or 25 cfm continuous
- Dryer duct length: 35 feet, less fitting allowances
- Dryer duct: 4-inch smooth metal, transition duct 8 feet maximum

## When is mechanical exhaust required in a house?

The 2021 International Residential Code (IRC) handles exhaust in Chapter 15, Sections M1501 through M1507. Three rules drive most dwelling designs:

- **Bathrooms without a window need a fan.** IRC Section R303.3 requires bathrooms and water closet compartments to have at least 3 square feet of glazing, half of it openable, unless artificial light and a local exhaust system are provided. That exhaust must go directly outdoors.
- **Exhaust always goes outdoors.** Section M1501.1 prohibits exhausting air into an attic, soffit, ridge vent or crawl space, and Section M1505.2 prohibits recirculating bathroom and toilet room air within the house or to another dwelling unit.
- **Fans must deliver the rated flow.** Section M1505.3 requires exhaust fans to be listed and labeled for airflow under ANSI/AMCA 210-ANSI/ASHRAE 51, and the bathroom rate must be met at a static pressure of at least 0.25 inch water column (Table M1505.4.4, note a).

Table M1505.4.4 sets the local exhaust capacity: 50 cfm intermittent or 20 cfm continuous for bathrooms and toilet rooms, and 100 cfm intermittent or 25 cfm continuous for kitchens. Section M1503 regulates domestic cooking exhaust where it is provided, and where a ducted range hood or downdraft system is installed its fan must meet the kitchen rate (Section M1503.5). Duct runs for bath and kitchen fans must stay within Table M1504.2 unless the installed airflow is verified, and each elbow deducts 15 feet of allowable length (note c).

## What are the clothes dryer exhaust duct requirements?

Dryer exhaust is the most frequently inspected exhaust item because lint-laden ducts are a fire risk. The IRC (Section M1502) and the International Mechanical Code (IMC, Section 504) set nearly identical rules:

- **Independent duct to the outdoors** with a backdraft damper and no screen at the termination (M1502.2, M1502.3; IMC 504.4).
- **Termination location** follows the dryer manufacturer's instructions; where they are silent, at least 3 feet in any direction from openings into the building, including ventilated soffits (M1502.3; IMC 504.4.1). The terminal must keep an open area of at least 12.5 square inches.
- **4-inch nominal smooth-wall metal duct**, at least 0.0157 inch thick (No. 28 gage) in the IRC and 0.016 inch in the IMC (M1502.4.1; IMC 504.9.1). Screws or fasteners cannot protrude more than 1/8 inch into the duct.
- **Supports** at intervals of not more than 12 feet in the IRC and at 4-foot intervals in the IMC (M1502.4.2; IMC 504.9.2).
- **Transition duct** a single listed length to UL 2158A, not more than 8 feet long and never concealed in construction (M1502.4.3).
- **Maximum length** of 35 feet from the transition duct connection to the outlet terminal, reduced for each fitting by the equivalent lengths in Table M1502.4.6.1 (IMC Table 504.9.4.1). The dryer manufacturer's instructions, or a UL 705 dryer exhaust duct power ventilator, can set a different limit. Booster fans are prohibited.
- **Length label** within 6 feet of the duct connection wherever the equivalent length exceeds 35 feet (M1502.4.7).
- **Shield plates** of steel at least 0.062 inch thick where the duct is less than 1-1/4 inches from the face of framing (M1502.5).

In buildings under the IMC, Section 504.7 adds makeup air for installations exhausting more than 200 cfm and a closet opening of at least 100 square inches where a dryer sits in a closet. Section 504.2 bans fire dampers in dryer ducts and limits penetrations of fire-resistance-rated assemblies.

## What do range hoods and kitchen exhaust require?

A domestic range hood must discharge outdoors through a smooth, airtight duct with a backdraft damper, independent of all other exhaust systems, and cannot terminate in an attic, crawl space or other area inside the building (IRC M1503.3). Ducts are galvanized steel, stainless steel or copper in the IRC (M1503.4); the IMC also allows aluminum (IMC 505.3). Schedule 40 PVC is allowed only for downdraft systems under a slab on grade, fully backfilled and solvent cemented. Listed ductless hoods are allowed where other ventilation is provided.

Makeup air is where the two codes differ. IMC Section 505.4 requires makeup air approximately equal to the exhaust rate for any domestic hood system able to exhaust more than 400 cfm, with a means of closure automatically controlled to operate with the hood. IRC Section M1503.6 requires it above 400 cfm only where a gas, liquid or solid fuel-burning appliance that is neither direct-vent nor mechanically vented sits inside the dwelling unit's air barrier, using a gravity damper or an electrically operated damper that opens automatically when the hood runs. Commercial kitchen hoods follow IMC Sections 506 through 509.

## Where can exhaust outlets terminate?

The IRC sets dwelling exhaust terminations in Section M1504.3: at least 3 feet from property lines, 3 feet from gravity air intakes, operable windows and doors, and 10 feet from mechanical air intakes unless the exhaust is at least 3 feet above the intake. The IMC sets three tiers in Section 501.3.1, summarized in the reference tables below. IMC exhaust openings other than dryer terminations are protected with corrosion-resistant screens, louvers or grilles with openings from 1/4 inch to 1/2 inch (Section 501.3.2).

## How are exhaust rates set in commercial buildings?

IMC Table 403.3.1.1 lists an exhaust airflow rate for spaces that generate odors or contaminants, alongside the outdoor air rates for each occupancy. Toilet rooms, locker rooms, kitchens, parking garages and similar spaces marked in the table require mechanical exhaust, and their air may not be recirculated to other spaces (notes b and g). Where a slash rate is shown, the higher number applies to intermittent operation and the lower number only where the system runs continuously while the space is occupied (notes e and f).

## Dryer exhaust duct length worksheet

Equivalent lengths are reproduced from IRC 2021 Table M1502.4.6.1, which matches IMC 2021 Table 504.9.4.1. Subtract each fitting from the 35-foot maximum; the transition duct is not counted. Typical model-code values; verify against the AHJ-adopted edition and the dryer manufacturer's instructions.

| Fitting | Equivalent length |
| --- | --- |
| 4-inch radius mitered 45-degree elbow | 2 ft 6 in |
| 4-inch radius mitered 90-degree elbow | 5 ft |
| 6-inch radius smooth 45-degree elbow | 1 ft |
| 6-inch radius smooth 90-degree elbow | 1 ft 9 in |
| 8-inch radius smooth 45-degree elbow | 1 ft |
| 8-inch radius smooth 90-degree elbow | 1 ft 7 in |
| 10-inch radius smooth 45-degree elbow | 9 in |
| 10-inch radius smooth 90-degree elbow | 1 ft 6 in |

| Duct route (worked by Harth) | Fitting deductions | Maximum straight duct remaining |
| --- | --- | --- |
| Two 6-inch radius smooth 90-degree elbows | 2 x 1 ft 9 in = 3 ft 6 in | 31 ft 6 in |
| Two 6-inch radius smooth 90s and one 6-inch radius smooth 45 | 3 ft 6 in + 1 ft = 4 ft 6 in | 30 ft 6 in |
| Two 4-inch radius mitered 90-degree elbows | 2 x 5 ft = 10 ft | 25 ft |
| Three 4-inch radius mitered 90-degree elbows | 3 x 5 ft = 15 ft | 20 ft |
| Four 10-inch radius smooth 90-degree elbows | 4 x 1 ft 6 in = 6 ft | 29 ft |

Using long-radius smooth elbows instead of mitered ones recovers up to 3 ft 6 in of duct per 90-degree turn.

## Minimum local exhaust rates in dwellings

| Space | IRC 2021 Table M1505.4.4 (one- and two-family) | IMC 2021 Table 403.3.1.1 (private dwellings) |
| --- | --- | --- |
| Kitchen | 100 cfm intermittent or 25 cfm continuous | 100 cfm intermittent or 50 cfm continuous |
| Bathroom or toilet room | 50 cfm intermittent or 20 cfm continuous | 50 cfm intermittent or 25 cfm continuous |
| Garage common to multiple units | Not listed | 0.75 cfm per sq ft |

## IMC 2021 Table 403.3.1.1 exhaust rates for common commercial spaces

Rates are per square foot of net occupiable floor area unless noted. Where two values are shown, the higher is for intermittent operation and the lower for continuous operation while occupied.

| Space | Exhaust rate |
| --- | --- |
| Toilet rooms, public | 70 cfm intermittent or 50 cfm continuous per water closet or urinal |
| Bathrooms and toilets, hotel and motel private | 50 cfm intermittent or 25 cfm continuous per room |
| Shower rooms | 50 cfm intermittent or 20 cfm continuous per showerhead |
| Kitchens (cooking), food and beverage service | 0.7 cfm per sq ft |
| Locker and dressing rooms, education | 0.25 cfm per sq ft |
| Sports locker rooms | 0.5 cfm per sq ft |
| Retail dressing rooms | 0.25 cfm per sq ft |
| Science laboratories | 1.0 cfm per sq ft |
| Art classrooms | 0.7 cfm per sq ft |
| Wood and metal shops | 0.5 cfm per sq ft |
| Copy and printing rooms | 0.5 cfm per sq ft |
| Darkrooms | 1.0 cfm per sq ft |
| Elevator car | 1.0 cfm per sq ft |
| Barber shops | 0.5 cfm per sq ft |
| Beauty and nail salons | 0.6 cfm per sq ft, plus 50 cfm source capture per nail station |
| Pet shops, animal areas | 0.9 cfm per sq ft |
| Repair garages and enclosed parking garages | 0.75 cfm per sq ft |
| Motor fuel-dispensing stations | 1.5 cfm per sq ft |
| Embalming rooms | 2.0 cfm per sq ft |
| Correctional cells with plumbing fixtures | 1.0 cfm per sq ft |

## Exhaust outlet termination clearances

| Exhaust type | Property line | Openings into buildings | Other |
| --- | --- | --- | --- |
| Dwelling exhaust (IRC M1504.3) | 3 ft | 3 ft from gravity intakes, operable windows and doors | 10 ft from mechanical air intakes, or 3 ft above them |
| Clothes dryer (IRC M1502.3, IMC 504.4.1) | Not specified | 3 ft in any direction, including ventilated soffits, unless the manufacturer specifies otherwise | Backdraft damper, no screen |
| Environmental air, IMC 501.3.1 item 3 | 3 ft | 3 ft from operable openings (other than Group U) | 10 ft from mechanical air intakes |
| Other product-conveying outlets, IMC 501.3.1 item 2 | 10 ft | 10 ft from operable openings | 3 ft from exterior walls and roofs; 10 ft above grade |
| Explosive or flammable vapors, fumes or dusts, IMC 501.3.1 item 1 | 30 ft | 10 ft from operable openings | 6 ft from walls and roofs; 30 ft from combustible walls and openings in the discharge direction; 10 ft above grade |

## Where can you explore related guidance?

- [Explore all design standards guidance](https://harth.build/resources/design-standards.md)
- [ADA Restroom Dimensions & Clearances (Typical Minimums)](https://harth.build/resources/design-standards/ada-restroom-dimensions.md)
- [Minimum Plumbing Fixture Requirements by Occupancy (IPC Table 403.1)](https://harth.build/resources/design-standards/plumbing-fixture-requirements.md)
- [Minimum Ceiling Height Requirements (IBC & IRC)](https://harth.build/resources/design-standards/minimum-ceiling-height-requirements.md)
- [Air, Exhaust: Building Code Definition](https://harth.build/resources/glossary/air-exhaust.md)
- [Ventilation: Building Code Definition](https://harth.build/resources/glossary/ventilation.md)

## Sources and method

Dwelling requirements are taken from [2021 International Residential Code Chapter 15, Exhaust Systems](https://codes.iccsafe.org/content/IRC2021P2/chapter-15-exhaust-systems) (Sections M1501 to M1505 and Tables M1502.4.6.1, M1504.2 and M1505.4.4) and [Chapter 3](https://codes.iccsafe.org/content/IRC2021P2/chapter-3-building-planning) (Section R303.3). Commercial and multifamily requirements come from the [2021 International Mechanical Code, Chapter 5, Exhaust Systems](https://codes.iccsafe.org/content/IMC2021P2/chapter-5-exhaust-systems) (Sections 501, 504 and 505) and [Chapter 4, Ventilation](https://codes.iccsafe.org/content/IMC2021P2/chapter-4-ventilation) (Table 403.3.1.1). The duct-length worked examples are calculated by Harth. These are typical model-code values; verify every rate, length and clearance against the edition adopted by the authority having jurisdiction, the applicable energy code and local amendments.

### Source links

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IRC2021P2/chapter-15-exhaust-systems)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IRC2021P2/chapter-3-building-planning)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IMC2021P2/chapter-5-exhaust-systems)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IMC2021P2/chapter-4-ventilation)

## Frequently asked questions

### Is a bathroom exhaust fan required by code?

Under IRC Section R303.3 a bathroom needs either 3 square feet of glazing with half openable, or artificial light plus a local exhaust fan discharging directly outdoors. The fan must deliver 50 cfm intermittent or 20 cfm continuous at 0.25 inch water column. That air may not be recirculated within the house (M1505.2).

### How long can a dryer vent be?

IRC Section M1502.4.6.1 and IMC Section 504.9.4.1 limit a 4-inch dryer exhaust duct to 35 feet from the transition duct connection to the outlet terminal, minus the equivalent length of each fitting, such as 5 feet per 4-inch radius mitered 90-degree elbow. The dryer manufacturer's instructions or a listed power ventilator can allow a longer run.

### How far must a dryer vent be from a window or door?

Where the dryer manufacturer's instructions do not set a location, the duct must terminate at least 3 feet in any direction from openings into the building, including ventilated soffits (IRC M1502.3, IMC 504.4.1). The termination needs a backdraft damper, no screen, and an open area of at least 12.5 square inches.

### When does a range hood need makeup air?

IMC Section 505.4 requires makeup air at about the exhaust rate for domestic hoods able to exhaust more than 400 cfm, with a closure controlled to operate with the hood. In one- and two-family dwellings, IRC Section M1503.6 applies the 400 cfm trigger only where a non-direct-vent, naturally vented fuel-burning appliance is inside the air barrier.

### Can a bathroom fan vent into the attic or soffit?

No. IRC Sections M1501.1 and M1505.2 require bathroom, toilet room and kitchen exhaust to discharge directly outdoors and prohibit discharge into an attic, soffit, ridge vent, crawl space or other area inside the building. The outlet must be at least 3 feet from property lines, operable windows and doors.

### How much exhaust does a public restroom need?

IMC Table 403.3.1.1 requires 70 cfm per water closet or urinal where the exhaust runs intermittently, or 50 cfm per fixture where it runs continuously while the space is occupied. A room with three water closets and two urinals therefore needs 350 cfm intermittent or 250 cfm continuous.

---

- Continue in Harth (open this topic in the Harth agent): https://app.harth.build/?context=%2Fresources%2Fdesign-standards%2Fmechanical-exhaust-requirements%2F
- View this page in a browser: https://harth.build/resources/design-standards/mechanical-exhaust-requirements/
- All resources for agents: https://harth.build/llms.txt