How long can a dryer vent be?
A dryer vent can be at most 35 feet long under the International Residential Code, measured from where the hose behind the dryer connects to the duct to the outlet outside, and every elbow uses up part of that 35 feet: a 4-inch radius mitered 90-degree elbow counts as 5 feet. The hose is not counted, but it can be no longer than 8 feet. A longer run is allowed only when the dryer maker's installation instructions allow it or when a dryer exhaust duct power ventilator that conforms to UL 705 sets the length. The 2021 and 2024 editions of the code say the same thing, in sections M1502.4.6.1 to M1502.4.6.3.
Maximum dryer vent length: the code numbers
| Part of the vent | Limit | Section |
|---|---|---|
| Exhaust duct (everything from the hose connection to the outlet terminal) | 35 feet, minus the equivalent length of each fitting | M1502.4.6.1 |
| Transition duct (the hose behind the dryer) | One listed piece, 8 feet at most, not concealed; not counted in the 35 feet | M1502.4.3, M1502.4.6.1 |
| Longer run, by the dryer maker | The size and maximum length in the dryer's installation instructions | M1502.4.6.2 |
| Longer run, with a power ventilator | The length in the ventilator maker's instructions; the ventilator must conform to UL 705 | M1502.4.6.3, M1502.4.4 |
| Booster fans | Not allowed | M1502.4.5 |
| Equivalent length over 35 feet | A permanent label or tag stating it, within 6 feet of the exhaust duct connection | M1502.4.7 |
| Duct size and material | 4 inches nominal, smooth inside, metal at least 0.0157 inch thick (No. 28 gage) | M1502.4.1 |
How long can a dryer vent be?
Section M1502.4.6.1 of the International Residential Code (IRC) says the maximum length of the exhaust duct shall be 35 feet from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, that maximum is reduced by the amounts in Table M1502.4.6.1, and the transition duct is not part of the length. So the code does not set one fixed length: it sets 35 feet of equivalent length, and each elbow spends some of it. The U.S. Fire Administration (USFA) puts the same rule in plain words: the total length of the vent duct should not exceed 35 feet, "shorter if there are turns or bends." To add up your own run, use our dryer vent length calculator.
The 35 feet is one of three ways to set the limit. Section M1502.4.6 says the maximum length is determined by one of three methods: the specified length above, the dryer manufacturer's installation instructions (M1502.4.6.2), or the instructions for a dryer exhaust duct power ventilator (M1502.4.6.3).
Dryer vent length chart: what each elbow counts as
| Fitting | Counts as |
|---|---|
| 4-inch radius mitered 45-degree elbow | 2 feet 6 inches |
| 4-inch radius mitered 90-degree elbow | 5 feet |
| 6-inch radius smooth 45-degree elbow | 1 foot |
| 6-inch radius smooth 90-degree elbow | 1 foot 9 inches |
| 8-inch radius smooth 45-degree elbow | 1 foot |
| 8-inch radius smooth 90-degree elbow | 1 foot 7 inches |
| 10-inch radius smooth 45-degree elbow | 9 inches |
| 10-inch radius smooth 90-degree elbow | 1 foot 6 inches |
Elbow type makes a large difference. A 4-inch radius mitered 90-degree elbow counts as 5 feet, while a 6-inch radius smooth 90-degree elbow counts as 1 foot 9 inches. When the length is set by the dryer maker's instructions (M1502.4.6.2), the maker's own fitting calculations apply, and the code says to use this table in their absence.
How long is too long for a dryer vent?
Anything over 35 feet of equivalent length, unless the dryer maker's instructions allow more or a power ventilator that conforms to UL 705 sets the length. Is 20 feet too long for a dryer vent? No: twenty feet of straight duct is 15 feet under the code limit, and it stays within the limit with up to three 4-inch radius mitered 90-degree elbows. These examples use only the code's numbers, with the hose behind the dryer left out, as the code says:
| Run | Equivalent length | Within 35 feet? |
|---|---|---|
| 20 feet of straight duct, no elbows | 20 feet | Yes, 15 feet to spare |
| 20 feet plus two 4-inch radius mitered 90-degree elbows | 20 + 5 + 5 = 30 feet | Yes, 5 feet to spare |
| 20 feet plus two 6-inch radius smooth 90-degree elbows | 23 feet 6 inches | Yes |
| 20 feet plus three 4-inch radius mitered 90-degree elbows | 35 feet | Yes, exactly at the limit |
| 25 feet plus three 4-inch radius mitered 90-degree elbows | 40 feet | No, 5 feet over |
A run that comes out over 35 feet is not automatically a violation. It needs the dryer maker's installation instructions to allow that length, or a power ventilator that conforms to UL 705, and then a permanent label or tag within 6 feet of the exhaust duct connection stating the equivalent length (M1502.4.7).
How many elbows can a dryer vent have?
The code sets no maximum number of elbows; it charges each one against the 35 feet instead. With 4-inch radius mitered 90-degree elbows at 5 feet each, four elbows use 20 feet and leave 15 feet for straight duct.
Dryer makers can count elbows differently. Whirlpool's venting guide says that when a model's installation instructions do not cover the number of elbows a job needs, the allowable vent system length drops by 10 feet for each extra 90-degree elbow and by 6 feet for each extra 45-degree elbow. Its example: a dryer allowed 40 feet of total vent length with two 90-degree bends is allowed 30 feet with three. That deduction applies to Whirlpool's rated length, not to the code's 35 feet.
How long can a dryer vent hose be?
Eight feet at most. The code calls the hose behind the dryer the transition duct, and section M1502.4.3 says it must be a single length, listed and labeled to UL 2158A, no longer than 8 feet, and not concealed within construction. It does not count toward the 35 feet. The USFA summarizes the rule the same way: not longer than eight feet, not concealed within construction, and listed and labeled to UL 2158A.
Whirlpool's instructions say not to install a dryer with flexible plastic venting or flexible metal (foil type) duct, and that any flexible metal duct used must be a type the appliance maker identifies as suitable for dryers. It says flexible venting materials are known to collapse, be easily crushed, and trap lint. The USFA adds that flexible vents can twist, allowing lint to build up and catch fire if it comes in contact with enough heat. Swapping a plastic or foil hose for a listed one is covered in dryer vent installation and replacement.
How long can a dryer vent be without a booster?
Without any fan, the limit is 35 feet of equivalent length, or the length the dryer maker's instructions allow. A booster fan is not a way around it: section M1502.4.5 says domestic booster fans shall not be installed in dryer exhaust systems. What the code does allow is a domestic dryer exhaust duct power ventilator that conforms to UL 705, installed to its maker's instructions (M1502.4.4). With one, the maximum duct length is whatever the ventilator maker's instructions say (M1502.4.6.3).
Whirlpool's venting guide says "Do not install a booster fan in the exhaust duct," with an exception for dryers in an engineered multiple-dryer system. It also says there is currently no UL classification for booster fans and their direct connection to dryers (to make sure the dryer will not run if the booster fan fails), so Whirlpool does not provide recommendations for booster fan use.
Can a dryer vent be too long?
Yes, and a vent that is too long collects lint. The USFA says that if a dryer vent is too long or has many bends and turns, moisture in the warm air passing through it condenses on the vent surfaces and attracts lint, which eventually accumulates and creates resistance. It notes that laundry rooms in upstairs bedrooms, hallways, bathrooms, kitchens and closets may need longer ducts to reach an outside wall, and says this makes regular inspection and cleaning crucial.
Some dryers report it. Whirlpool says certain electronic models detect airflow, and on a single-dryer vent an "AF" code means the vent may be blocked or partially blocked, or the overall vent system length is too long. Its steps: check that the duct is not crushed, clear lint from the duct and the outside hood, confirm the run length and number of elbows are within the recommended limits, and have the entire vent run cleaned if the message persists. Whirlpool says the air in the duct should move at least 1,200 feet per minute to keep lint moving, which in a 4-inch pipe takes at least 105 CFM of airflow. The other warning signs are on our page about the signs of a clogged dryer vent, and the cleaning schedule is on how often to clean a dryer vent.
What is the longest a dryer vent can be?
Whatever the dryer's installation instructions allow, if they allow more than 35 feet. Under M1502.4.6.2 the dryer manufacturer's installation instructions set the size and maximum length of the duct, and the code official is to be given a copy of the instructions for the make and model at the concealment inspection. Whirlpool says each of its models has a maximum rated vent length, shown in the product literature supplied with it or on Whirlpool.com, and its guide refers to "long vent dryers." The back pressure chart in its venting guide has rows for maximum rated vent lengths, without 90-degree elbows, of 36 to 37 feet, 64 feet, 100 feet, 120 feet and 130 feet. Check the rating for your exact model before planning a long run, or have a dryer vent company measure it.
Dryer vent length code: 2021 and 2024 IRC and state versions
The 2021 and 2024 IRC number the dryer duct sections the same way, M1502.4.1 through M1502.4.8, with identical length wording. The 2024 edition adds section M1502.6: installations exhausting more than 200 cubic feet per minute must be provided with makeup air, and where a closet is designed for a dryer, makeup air follows the dryer maker's instructions or, if they give no specification, comes from permanent openings totaling at least 100 square inches or other approved means (M1502.6.1). The 2018 North Carolina code uses older numbering and puts the same length rule in M1502.4.5.1. States also amend these sections:
| Code | Length rule | Support and joint changes we found (the length rule is unchanged) |
|---|---|---|
| 2021 and 2024 IRC | 35 feet minus fittings (M1502.4.6.1) | The model code: ducts supported at intervals of no more than 12 feet (M1502.4.2) |
| 2018 North Carolina Residential Code | 35 feet minus fittings (M1502.4.5.1); dryer maker's instructions (M1502.4.5.2); power ventilator (M1502.4.5.3); label over 35 feet (M1502.4.6) | The transition duct must remain entirely within the room where the appliance is located (M1502.4.3). An approved amendment, with a delayed effective date of January 1, 2021, changes the support interval from 12 feet to 4 feet (M1502.4.2). |
| 2023 Oregon Residential Specialty Code | 35 feet minus fittings (M1502.4.6.1) | Ducts supported at intervals of no more than 4 feet, and joints sealed with listed tape (M1502.4.2); its transition duct section (M1502.4.3) has no UL 2158A wording |
| 2021 Virginia Construction Code, section 310.8 amendments to the IRC | Not amended; the IRC rule applies | Exhaust ducts supported at four-foot intervals (M1502.4.2) |
Your city may amend the code further. The code expects the dryer's installation instructions to be shown to the code official at the concealment inspection, so ask your building department whether rerouting a long run needs a permit.
Dryer vent length: common questions
How far can you run a dryer vent?
How long should a dryer vent be?
How long should a dryer vent hose be?
Can a dryer vent hose be too long?
Dryer vent 90 degree elbow
How far can you vent a dryer vertically?
Sources
- International Code Council, 2021 International Residential Code, Chapter 15, section M1502 Clothes dryer exhaust: codes.iccsafe.org
- International Code Council, 2024 International Residential Code, Chapter 15, section M1502 Clothes dryer exhaust: codes.iccsafe.org
- 2018 North Carolina State Building Code: Residential Code, Chapter 15: codes.iccsafe.org
- North Carolina Office of State Fire Marshal, approved amendments to the 2018 North Carolina Residential Code: www.ncosfm.gov
- North Carolina Office of State Fire Marshal, "North Carolina Delays Implementation of 2024 State Building Code": www.ncosfm.gov
- 2023 Oregon Residential Specialty Code, Chapter 15, Exhaust Systems: codes.iccsafe.org
- Virginia DHCD, 2021 Virginia Construction Code: www.dhcd.virginia.gov
- U.S. Fire Administration, "Clothes Dryer Fires in Residential Buildings (2008 to 2010)", August 2012: usfa.fema.gov
- Whirlpool, "Dryer venting specifications" (W11688180A): whirlpool.com