Dryer exhaust is one of the few residential systems where the code gives you a hard length budget and then charges you for every turn. Getting it right is mostly arithmetic, plus one 2021 change that catches remodelers.
Dryer exhaust length, elbows and termination — IRC 2021 M1502.Open full diagram →
Requirement
Model Code Standard
Section
Duct material
Exhaust duct is 4 in. nominal diameter, metal, with a smooth interior and a wall thickness of at least 0.0157 in. (No. 28 gage)
IRC M1502.4.1
Length budget
Maximum developed length is 35 ft from the dryer connection to the termination, before deductions
IRC M1502.4.6.1
Fitting deductions
Each fitting is deducted from the 35 ft according to Table M1502.4.6.1 — a 4 in. mitered 90-degree elbow costs 5 ft and a mitered 45 costs 2 ft 6 in., while smooth-radius fittings in larger diameters cost far less
IRC Table M1502.4.6.1
Transition duct
The flexible transition between the appliance and the duct may be up to 8 ft long, must be listed to UL 2158A, and may not be concealed in construction
IRC M1502.4.3
Termination
The outlet terminates outside the building with a backdraft damper and no screen, kept at least 3 ft from any other opening, including ventilated soffit openings
IRC M1502.3
Length label
Where the developed length exceeds 35 ft, a permanent label or tag identifying the equivalent length must be posted near the dryer connection
IRC M1502.4.7
Worth knowing
The 2021 edition prohibits ordinary domestic booster fans in dryer exhaust (M1502.4.5). Where a run needs help, the listed device is a dryer exhaust duct power ventilator listed to UL 705 under M1502.4.4, and installing one changes how the length limit is applied.
Sheet-metal screws are banned in dryer duct, but not by M1502. The 1/8 in. protrusion limit is the mechanical-chapter rule; the outright prohibition on screws sits in the fuel-gas chapter at G2439.3.
Makeup air and the louvered closet door are fuel-gas provisions too. G2439.5 requires makeup air where the dryer moves more than 200 cfm, and the 100 sq in. closet opening is G2439.5.1. Neither appears in M1502.
Two IRC chapters give different numbers for the same duct. M1502 sets the metal thickness at 0.0157 in. and support spacing at 12 ft; G2439 says 0.016 in. and 4 ft. Where a gas dryer is involved, expect the inspector to enforce the tighter pair.
California is not on 35 ft. The mechanical code in use there allows 14 ft of developed length including two 90-degree bends, with a deduction for each additional elbow.
Common questions
What does the code say about duct material for Dryer Vent?
Exhaust duct is 4 in. nominal diameter, metal, with a smooth interior and a wall thickness of at least 0.0157 in. (No. 28 gage). (IRC M1502.4.1)
What does the code say about length budget for Dryer Vent?
Maximum developed length is 35 ft from the dryer connection to the termination, before deductions. (IRC M1502.4.6.1)
What does the code say about fitting deductions for Dryer Vent?
Each fitting is deducted from the 35 ft according to Table M1502.4.6.1 — a 4 in. mitered 90-degree elbow costs 5 ft and a mitered 45 costs 2 ft 6 in., while smooth-radius fittings in larger diameters cost far less. (IRC Table M1502.4.6.1)
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.
This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.