How long can your dryer vent be?
Enter the straight run and the elbows, get the equivalent length against the 35 ft code limit.
Table M1502.4.6.1 counts each elbow as a length of straight duct. The IRC calls for 4-inch duct; the larger sizes are for a duct the dryer maker specifies.
From the dryer connection to the outside, not counting the short flexible transition duct.
Leave empty for the IRC's 35 ft. If the manual gives its own maximum length, type it here.
How long can a dryer vent be?
The International Residential Code (2021 and 2024 editions, section M1502.4.6.1) sets the maximum length of the exhaust duct at 35 feet, measured from the connection to the transition duct at the dryer to the outlet, and reduces it for every fitting you use (Table M1502.4.6.1). The short transition duct behind the dryer is not counted. If the dryer maker's installation instructions give another maximum length, the code lets them decide (M1502.4.6.2). The 2018 edition numbers the same rule M1502.4.5.1. Local codes can differ, so check with your building department.
How much does each elbow take off?
Table M1502.4.6.1 gives each fitting an equivalent length of straight duct. For 4-inch radius mitered elbows, a 45° elbow counts as 2 feet 6 inches and a 90° elbow as 5 feet. Smooth elbows with a larger radius count less: 1 foot and 1 foot 9 inches for 6-inch, 1 foot and 1 foot 7 inches for 8-inch, 9 inches and 1 foot 6 inches for 10-inch. Example: 20 feet of straight duct and two 90° mitered elbows count as 20 + 10 = 30 feet, which leaves 5 feet under the 35-foot limit: room for one more 90° elbow or two more 45° elbows.
What are the duct rules besides length?
The IRC asks for a smooth interior and metal at least 0.0157 inch thick (No. 28 gage), 4 inches in nominal diameter (M1502.4.1). The duct is supported at intervals of no more than 12 feet and secured, joints are sealed and mechanically fastened, and no screws or similar fasteners may stick more than 1/8 inch into the duct (M1502.4.2). The transition duct that joins the dryer to the duct is a single listed length (UL 2158A), no longer than 8 feet and not hidden in the construction (M1502.4.3). Domestic booster fans are prohibited (M1502.4.5). The 2021 International Mechanical Code has its own wording in section 504.9, with metal at least 0.016 inch thick and supports every 4 feet.
Where does the dryer duct have to end?
Outside the building. The termination follows the dryer maker's instructions; if they name no place, it must be at least 3 feet in any direction from openings into the building, including ventilated soffits (M1502.3). It needs a backdraft damper and no screen, and the passage must stay open, with at least 12.5 square inches of open area (M1502.3.1).
What if my vent is too long?
First read the dryer's installation manual: the code lets the maker's maximum length rule (M1502.4.6.2), and the inspector should get a copy for your make and model. Type that number into the limit field. If the equivalent length is above 35 feet, the IRC asks for it on a permanent label or tag within 6 feet of the duct connection (M1502.4.7). A listed duct power ventilator follows its own maker's instructions for length (M1502.4.6.3). Otherwise shorten the run, use fewer elbows or move the outlet. Follow your dryer manual and local code.
Sources: International Residential Code 2021 and 2024, sections M1502.3 to M1502.4.7 and Table M1502.4.6.1 (numbered M1502.4.5.1 in the 2018 edition), and International Mechanical Code 2021, section 504.9 and Table 504.9.4.1 (ICC, codes.iccsafe.org, free view, read October 7, 2026); the length check is arithmetic on that table. Airflow of a fan or hood: CFM calculator; whole-house ventilation: ventilation rate calculator; laying out other pipes: pipe slope calculator. How we check our calculators.