Dryer-vent developed length — 35-foot maximum minus 5 feet per 90-degree elbow and 2.5 feet per 45
A 4″ dryer duct is capped at 35 ft of developed length (IRC M1502.4.6.1) — but each 90° elbow costs 5 ft and each 45° costs 2.5 ft. Two 90s and a 45 leave 35 − 10 − 2.5 = 22.5 ft of straight duct. Foil-tape the joints (no screws — they snag lint); the wall cap gets a damper and no screen.
What this diagram shows
A diagram of a clothes-dryer exhaust route and its developed-length budget. Across the top, a dryer connects through a flexible transition duct of up to 8 feet, then a rigid smooth-interior metal duct runs through two 90-degree elbows and one 45-degree elbow to a wall cap with a backdraft damper and no screen. A formula reads: allowed length equals 35 feet minus 5 feet times the number of 90-degree elbows minus 2.5 feet times the number of 45-degree elbows, per IRC M1502.4.6.1 for a 4-inch duct. Below, a 35-foot budget bar is divided into a green segment of 22.5 feet of straight duct plus three amber segments for the two 90-degree elbows at 5 feet each and one 45-degree elbow at 2.5 feet, summing to 35. The result: 35 minus 10 minus 2.5 equals 22.5 feet of straight duct. Joints are foil-taped, not screwed, because screw tips snag lint.
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