How long can a dryer vent be?
Under Section M1502 of the International Residential Code, the maximum length of a dryer exhaust duct is 35 feet, measured from the connection at the transition duct to the outlet terminal. That length is then reduced for every fitting in the run, using the code's equivalent-length table — but the deduction is not a flat five feet per elbow. It depends on the elbow's radius and type, ranging from about 9 inches for a 10-inch radius 45-degree elbow up to 5 feet for a 4-inch radius mitered 90. Where the dryer manufacturer publishes its own equivalent lengths, those take precedence over the table.
The short version
- The base limit is 35 feet, from the transition duct connection to the outside terminal.
- Every fitting reduces it, but there is no flat "five feet per elbow" rule — the deduction depends on radius and type.
- A 4-inch radius mitered 90° elbow costs 5 feet. A 6-inch radius smooth 90° costs 1 foot 9 inches. Geometry matters.
- The manufacturer's published equivalent lengths override the code table where they exist.
- Duct must be metal, at least 28 gage, 4 inches nominal, with a smooth interior. White vinyl and ribbed foil flex fail on both counts.
- The terminal needs a backdraft damper and no screen. A screen is a lint trap in the worst possible place.
The 35-foot rule, and the part everyone gets wrong
Section M1502.4.5 of the International Residential Code sets the maximum exhaust duct length at 35 feet. That is measured from the point where the duct connects to the transition duct behind the dryer, out to the terminal. The flexible transition duct itself is not counted in the 35.
Then M1502.4.5.1 reduces it for fittings. This is where most of the advice online is simply wrong, because it repeats "deduct five feet per elbow" as though every elbow is the same. The code table does not say that:
| Fitting | Deduct |
|---|---|
| 4" radius mitered 45° elbow | 2 ft 6 in |
| 4" radius mitered 90° elbow | 5 ft |
| 6" radius smooth 45° elbow | 1 ft |
| 6" radius smooth 90° elbow | 1 ft 9 in |
| 8" radius smooth 90° elbow | 1 ft 7 in |
| 10" radius smooth 90° elbow | 1 ft 6 in |
A tight mitered 90 costs nearly three times what a wide smooth 90 costs. Two of the tight ones eat ten feet of your allowance; two of the smooth ones eat three and a half. On a marginal run that is the difference between compliant and not.
One more thing worth knowing: under M1502.4.5.2, where the dryer manufacturer publishes its own equivalent-length figures, those govern instead of the table. Check the manual before anyone quotes you the code.
Why the material matters as much as the length
M1502.4.1 requires the duct to be metal, at least 0.0157 inches thick (28 gage), 4 inches nominal diameter, with a smooth interior. White vinyl and ribbed foil accordion duct fail that on two separate counts: they are not metal, and their corrugated interior is the opposite of smooth.
That ribbing is not a cosmetic detail. Every corrugation is a ledge that lint catches on, which is why a foil run loads up far faster than a smooth metal one of the same length.
The Consumer Product Safety Commission's guidance is blunt: "Replace plastic or foil, accordion-type ducting material with rigid or corrugated semi-rigid metal duct."
The one flexible section the code does allow is the transition duct behind the dryer — and even that has to be a single length listed to UL 2158A, no longer than 8 feet, and never concealed inside construction.
The terminal
Under M1502.3 the exhaust has to terminate outside the building, at least 3 feet from any opening, with a backdraft damper and no screen.
The no-screen part surprises people, because a screen seems sensible. It is a lint trap positioned at the one point in the system where you least want one, and we pull matted lint off screened terminals constantly. If pests are the concern, the answer is a proper louvered or hooded termination designed for dryer exhaust, not hardware cloth.
What this has to do with fires
The National Fire Protection Association's figures, covering 2010 to 2014, put it at roughly 15,970 home fires a year involving clothes dryers and washing machines, with 92% of those involving dryers — causing 13 deaths, 440 injuries and $238 million in direct property damage annually.
The leading cause, at 31%, is failure to clean. And the item first ignited in 27% of dryer fires is dust, fiber or lint.
A long run with tight elbows in corrugated foil is a machine for accumulating exactly that material. The code limits are not bureaucratic — they are describing the conditions under which a dryer can actually clear its own lint.
One claim we are not going to make
You will read that cleaning your dryer vent cuts drying time by some percentage or saves a specific amount per year. We went looking for a primary source for those numbers and could not find one. The Department of Energy's guidance says long runs and excessive bends increase drying time, which is sound, but nobody we could find has published measured savings.
So we will tell you what is documented — the fire data, the code limits, the material requirements — and leave the invented numbers to everyone else.
What the fire data shows
Common questions
My dryer vent is longer than 35 feet. What now?
The code anticipates it. Where the equivalent length exceeds 35 feet, M1502.4.6 requires a permanent label within 6 feet of the duct connection stating the length, so whoever services the dryer knows. A booster fan is the usual engineered solution for a run that cannot be shortened.
How do I know what radius my elbows are?
Mostly by looking. A tight, sharply folded 90 that looks like two pieces crimped together is a mitered elbow. A wide sweeping bend is a smooth radius elbow. If the run is concealed, an inspection is how you find out what is actually up there.
Is the foil duct behind my dryer illegal?
Not necessarily. A single flexible transition duct behind the dryer is permitted if it is listed to UL 2158A, under 8 feet, and not concealed. What is not permitted is foil or vinyl accordion duct running through the wall, ceiling or attic as the actual exhaust duct.
How often should a dryer vent be cleaned?
It depends on run length, geometry, material and how much laundry the household does — a short rigid metal run in a two-person house is a completely different case from a 30-foot run with three elbows serving a family of six. That is why we look before quoting a schedule.
Can I just clean it myself?
You can do the lint trap housing and, if you can reach it, the terminal. The full run is a different job, and the common DIY failure is pushing a blockage further in or separating a joint inside a wall where you cannot see or reach it. We wrote about where the line is.
Sources and standards
Where this page relies on published guidance, here is what it relies on.
- NFPA — Home Fires Involving Clothes Dryers and Washing Machines (2017, 2010-2014 data) — National fire-incident estimates, leading causes and leading items first ignited.
- US CPSC — Publication 5022, Clothes Dryers — Federal safety guidance, including replacing plastic or foil accordion duct with metal.
- International Residential Code, Section M1502 — Clothes Dryer Exhaust — Code requirements for dryer duct material, diameter, maximum length and termination.
