Dryer Vent Length Calculator
Use this free dryer vent max length calculator to check whether your dryer vent run meets the building code limit. Enter your straight duct length and the number of 90° and 45° elbows, and get an instant pass/fail answer against the 25-foot IRC rule for 4-inch smooth rigid metal duct.
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Rule applies to 4-inch smooth rigid metal duct only. The short transition duct behind the dryer does not count toward the limit.
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How to use this calculator
Measure the straight length of rigid metal duct from the dryer location to the wall or roof termination, in feet, and enter it above. Then count every bend in the duct run: each 90° elbow and each 45° elbow. Offset bends made of two 45° fittings count as two 45° elbows. The short flexible transition hose behind the dryer does not count — only the fixed duct run.
The calculator instantly converts your elbows into equivalent straight-duct feet, shows your effective duct length, and tells you whether the run passes the code maximum. If it fails, it shows exactly how many feet too long your run is — so you can shorten the route, remove a bend, or check whether your dryer's manual allows a longer run.
How it works
Section M1502.6 of the International Residential Code (IRC) limits a clothes dryer exhaust duct to a maximum developed length of 25 feet for 4-inch smooth rigid metal duct. Every bend restricts airflow and adds lint-trapping surface, so the code deducts equivalent length for each elbow: 5 feet per 90° elbow and 2.5 feet per 45° elbow.
The math is simple:
Effective length = straight run + 5 × (90° elbows) + 2.5 × (45° elbows)
Max allowed straight run = 25 − 5 × (90° elbows) − 2.5 × (45° elbows)
A run passes when its effective length is 25 feet or less; the remaining margin is 25 minus the effective length. A run fails when effective length exceeds 25 feet, and the overage tells you how much to shorten the duct or how many bends to remove.
This is the generic code rule, not a substitute for your dryer manual. The code explicitly defers to the manufacturer's installation instructions — always check the dryer manual first, since many manufacturers publish their own, stricter or more generous, maximum vent lengths that override the 25-foot code figure.
Frequently asked questions
Can I use flexible foil duct for my dryer vent?
No. Code requires dryer exhaust ducts to be rigid metal with smooth interior surfaces. Flexible or foil duct is not code-compliant for concealed runs because its ribs trap lint and restrict airflow, creating a fire hazard. Only a short, UL 2158A-listed transition duct (up to 8 feet, exposed and not concealed in a wall) may connect the dryer to the main duct.
How often should I clean a dryer vent?
Clean your dryer vent at least once a year. Longer vent runs, runs with several elbows, and dryers that see heavy use collect lint faster and should be checked every six months. Lint buildup is one of the leading causes of dryer fires, so regular cleaning matters for safety as well as drying speed.
Does a booster fan help with long dryer vent runs?
Yes, a dryer duct booster fan can help move air through runs that exceed the standard length limit, but only if the dryer manufacturer allows it. Check your dryer's manual first: the manufacturer's installation instructions override the generic code maximums, and some dryers explicitly permit (or prohibit) booster fans.
Can I use 3-inch duct instead of 4-inch for a dryer vent?
No. The IRC requires a 4-inch nominal diameter for dryer exhaust ducts, and dryers are designed to move air through 4-inch duct. A 3-inch duct would dramatically increase air resistance and lint buildup, and it does not meet code.