Most residential dryers need a 4-inch diameter vent hose, matched to a 4-inch exhaust duct and outdoor vent cap.
For the full breakdown, see our best Dryer Vent Hose guide.
The answer to what size dryer vent hose you need is almost always 4 inches. Nearly every residential dryer made for the US market ships with a 4-inch exhaust outlet, and the vent system—hose, duct, and exterior cap—should stay at that same 4-inch diameter from the dryer all the way to the outdoors. Shrinking that run to 3 inches or necking it down mid-route invites lint buildup and airflow restriction, which are exactly the conditions that lead to clogs and dryer fires.
The single decision that matters most: keep the whole path 4 inches, use smooth metal, and keep the run short. If you’re replacing an old hose or installing a new dryer, here are the rules that apply today.
Sizing Rules: Duct Diameter, Length, and Fittings
The 4-inch rule covers the full exhaust path: the hose connecting the dryer to the wall, the rigid duct inside the wall, and the vent cap outside. They all need to match at 4 inches nominal diameter.
Length is the second limit. Building guidance for US residential installations caps the total calculated duct run at 35 feet. Every fitting adds “virtual” length you must subtract from that budget:
- 45-degree bend: subtract 2.5 feet from your 35-foot allowance
- 90-degree bend: subtract 5 feet
- Every foot of actual duct counts as one foot
A run with two 90-degree turns effectively carries a 10-foot penalty, leaving only 25 feet for the straight sections. If your laundry room layout pushes past the limit, the fix is relocating the dryer or the vent outlet—not squeezing the duct to a smaller size.
The vent termination matters too. The exterior hood must open fully when the dryer runs, and the cap itself should match the 4-inch duct size. A partially blocked or undersized cap strangles airflow even when the interior duct is perfect.
Fitting Types: What the Code Allows and What It Rejects
Not all vent hose is created equal, and the material you choose is a fire-safety decision. The code-compatible answer is rigid or semi-rigid metal duct with a smooth interior finish. That smooth interior gives lint nothing to cling to and keeps airflow at full strength.
Flexible duct is the common offender. It’s cheap and easy to wrestle into tight spots, but its ribbed interior traps lint, and it kinks and crushes easily. If you use a short piece of foil transition duct to connect the dryer to the wall pipe, keep it visible, keep it as straight as possible, and only if the dryer manufacturer and your local code allow it.
Common Sizing Mistakes and Fire Hazards
Three mistakes cause the vast majority of dryer vent problems, and all of them are sizing or material errors:
- Using a 3-inch vent or reducing the diameter mid-run. This is a clogging and fire hazard, and it violates code. The dryer is designed to push air through a 4-inch path.
- Long, crushed, or kinked flexible hoses. Each kink is a choke point where lint piles up. A crushed hose can cut airflow by half or more.
- Concealing flexible duct in construction. If you can’t see it, you can’t clean it, and lint will still accumulate inside the ribs.
Dryer venting is a fire-risk system. If you’re buying new materials, a straight 4-inch metal run with minimal bends is the safest possible setup.
FAQs
Can I use a 3-inch dryer vent hose if my dryer outlet is 4 inches?
No. Reducing the diameter from 4 inches to 3 inches mid-run is both a code violation and a safety hazard. The smaller pipe restricts airflow, which causes longer drying times, higher heat buildup, and faster lint accumulation. The dryer’s exhaust outlet is sized to match a 4-inch duct, and the entire path to the outdoors should stay at that diameter.
How long should a dryer vent hose be?
Keep the total calculated length at 35 feet or less, counting deductions for every bend. A 90-degree turn subtracts 5 feet from your allowance, and a 45-degree turn subtracts 2.5 feet. Shorter is always better: a longer run means more surface area for lint to collect and more resistance to airflow.
Is flexible foil dryer hose safe to use?
Flexible foil duct is acceptable only for the short visible connection between the dryer and the wall pipe, and only if the manufacturer and local code permit it. It should never be concealed inside walls, floors, or ceilings. The ribbed interior catches lint, and the material kinks and crushes easily, so a rigid or semi-rigid smooth metal duct is the safer choice for the main run.