The maximum length depends on the type of duct and how many turns it makes
A dryer vent hose can run up to 25 feet in a straight line, but that distance shrinks with every turn the duct makes. Most building codes allow 25 feet of straight duct, but subtract 2.5 feet for each 45-degree bend and 5 feet for each 90-degree turn. So a duct that runs 15 feet straight and then makes two 90-degree turns has only 5 feet of effective length left before your dryer will start taking longer to dry clothes and lint will build up inside the hose.
The reason is airflow. Your dryer pushes hot, moist air and lint out through the duct to the outside. The longer and more twisted the path, the harder the dryer's fan has to work to push that air out. When the duct gets too long or too convoluted, the fan can't overcome the resistance, and wet lint starts collecting inside instead of exiting. This is a fire hazard and also makes your dryer run hot and fail sooner.
Key Takeaways
- A dryer vent hose can be 25 feet maximum in a straight line, but each 90-degree bend reduces that by 5 feet and each 45-degree bend by 2.5 feet.
- Rigid metal duct moves air better than flexible foil hose, so you can run rigid duct longer and with more bends before hitting the limit.
- Lint buildup inside a long or kinked duct is a fire risk and forces your dryer to work harder and wear out faster.
- The shortest path outside is always better—if you can move the dryer closer to an exterior wall, do it.
Why duct length matters to your dryer's performance
Every foot of duct and every bend adds friction that resists the airflow your dryer is trying to push out. Your dryer's blower fan has a fixed amount of power. When the duct is short and straight, the fan easily moves warm air and lint particles out to the outside. When the duct is long, has multiple turns, or is partially clogged with lint, the fan has to work much harder.
If the duct is too long or too restrictive, the fan can't push the air out fast enough. Lint settles inside the duct instead of exiting. Over time, lint accumulates and blocks more airflow, which makes the problem worse. The dryer then runs hotter and longer to try to dry the same load, which wears out the heating element and thermal fuse faster. A clogged vent is also the leading cause of dryer fires.
Rigid metal duct versus flexible foil hose
Rigid metal duct is smooth inside, so air moves through it with less friction. You can run rigid duct longer and make more turns before you hit the effective length limit. Most building codes treat rigid duct as the standard and allow the full 25 feet plus bends. Rigid duct also resists crushing and kinking, which keeps the inside diameter open.
Flexible foil hose (the accordion-style duct you see in many homes) has ridges inside that create more friction. Air doesn't move through it as easily, so you should treat it as having a shorter effective length. Some codes allow flexible hose only up to 8 feet, or require it only as a transition between the dryer and the wall—not as the main run to the outside. Flexible hose also kinks easily, which blocks airflow even more.
If your dryer is connected with flexible hose and it's more than 8 feet to the outside, consider replacing the flexible section with rigid duct. This is one of the most effective ways to improve dryer performance and reduce fire risk.
How to measure your duct run and calculate effective length
Start at the dryer outlet and trace the path the duct takes to the outside. Measure the straight sections in feet. Then count the bends: note how many are 90-degree turns (like a corner) and how many are 45-degree angles (gentler bends).
Use this formula: Start with 25 feet. Subtract 5 feet for each 90-degree bend. Subtract 2.5 feet for each 45-degree bend. The number you end up with is your effective duct length. If you get a negative number, your duct run is too long and restrictive for safe operation.
For example: 12 feet straight + one 90-degree turn + one 45-degree turn = 12 − 5 − 2.5 = 4.5 feet of effective length remaining. This is still acceptable. But 20 feet straight + two 90-degree turns = 20 − 10 = 10 feet of effective length, which is also fine. However, 25 feet straight + three 90-degree turns = 25 − 15 = 10 feet, which is acceptable but leaves no room for lint buildup.
Signs your vent hose is too long or clogged
If your dryer is taking longer than 45 minutes to an hour to dry a normal load, the duct is probably too restrictive. The dryer may also feel very hot to the touch on the outside, or the air coming out of the vent on the outside wall may feel weak or barely warm. These are signs the fan is struggling against too much resistance.
Another sign is lint visible around the dryer or on the floor near it. If lint is escaping around the door seal or the connection points, it means the pressure inside the dryer is building up because air can't exit fast enough. You may also notice a burning smell, which means the lint inside the duct is getting hot.
The most reliable check is to go outside and feel the air coming out of the vent hood. It should be warm and forceful. If it's barely moving or cool, your duct is too long, too clogged, or both.
Shortening a vent run that's too long
The best solution is to move the dryer closer to an exterior wall. This is not always possible in an existing home, but if you're renovating or installing a new dryer, placing it near the outside wall is the single most important decision you can make for vent performance.
If moving the dryer is not an option, replace flexible hose with rigid metal duct. Rigid duct has less friction, so you can run it longer. You can also reduce the number of bends by rerouting the duct—for example, running it straight up through a wall cavity instead of around obstacles. Every bend you eliminate adds back effective length.
Some people install a duct booster fan (also called a duct damper fan or inline fan) inside the duct to help push air out. These are small fans that mount inside the duct and set up when the dryer runs. They can add 10 to 15 feet of effective length. However, they add cost and another component that can fail, so they are a last resort if shortening the duct is truly impossible.
Cleaning and maintaining your vent hose
Even a duct that is the right length will perform poorly if it's clogged with lint. Clean the lint trap after every load—this is the first line of defense. But lint also escapes into the duct itself, especially if the trap is worn or not seated properly.
Once or twice a year, disconnect the duct from the back of the dryer and use a dryer vent brush (a long, flexible brush with bristles designed for this job) to pull lint out from inside. You can also use a shop vacuum with a hose attachment. Work from the dryer end toward the outside, pushing lint out the vent hood.
If you can't reach the entire duct or if lint is packed tightly inside, call a professional duct cleaning service. They have equipment that can clean the entire run from end to end. This is worth doing if your dryer is slow or if you haven't cleaned the duct in several years.
Frequently Asked Questions
Can I use a duct that's 30 feet long if it's straight with no bends?
No. Building codes set 25 feet as the maximum for straight duct, even with no bends. Beyond that, the friction inside the duct itself becomes too much for the dryer fan to overcome. If your dryer is more than 25 feet from the outside, you need to either move the dryer or install a duct booster fan.
Does the type of vent hood on the outside affect how long the duct can be?
Yes. A vent hood with a damper (a flap that opens when the dryer runs and closes when it stops) adds a small amount of resistance. A hood that's clogged with lint or bird nesting adds much more. Make sure the outside hood is clean and the damper moves freely. A damaged or missing damper should be replaced.
What if my dryer is in the basement and the vent goes up through the roof?
Vertical runs are harder on the dryer than horizontal runs because the fan has to push air upward against gravity. Treat vertical duct as having a shorter effective length—subtract an extra 5 feet from your total. So a basement dryer with 20 feet of vertical duct plus one 90-degree turn would be 20 − 5 (for the turn) − 5 (for vertical) = 10 feet of effective length.
Is flexible hose ever acceptable for the full vent run?
Most codes allow flexible hose only as a short transition (usually 8 feet or less) between the dryer and a wall or rigid duct. Using flexible hose for the entire run to the outside is not recommended because it restricts airflow too much and kinks easily. If your dryer is connected with flexible hose all the way to the outside, replacing at least the long section with rigid duct will improve performance significantly.
How do I know if my duct booster fan is working?
A duct booster fan should set up when the dryer starts and you should hear a slight humming sound from inside the duct. Feel the air at the outside vent hood—it should be noticeably warmer and stronger than before the fan was installed. If the fan doesn't set up or the airflow doesn't improve, the fan may be faulty and should be replaced.