Ducts, pipes & wiring · Dryer Vent Length Checker

Dryer vent length with two 90° elbows

With two 90° elbows of the 4-inch-radius mitered kind, the 2021 IRC 35 ft method leaves 25 ft of straight duct. Change the numbers below to check your own run.

Short answer

Longest straight duct allowed: 25 ft
2 mitered 90° elbows count as: 10 ft
Limit under the 2021 IRC 35 ft method: 35 ft

For 4-inch-radius mitered elbows under 2021 IRC M1502.4.6.1. The code also lets you size the duct by the dryer manual instead, and other editions and local amendments can differ.

Your duct run

Total straight duct, in feet

From the wall connection to the outside hood. A plain number is feet; you can also type 22' 6". Leave out the flexible connector behind the dryer.

Standard elbows

90° elbows

45° elbows

The code table calls these 4-inch-radius mitered elbows; for another kind, check the list below.

Long-radius smooth elbows

90°, 6" radius

45°, 6" radius

90°, 8" radius

45°, 8" radius

90°, 10" radius

45°, 10" radius

Which maximum are you working to?

The code's 35 ft methodDryer manual: its chart gives a maximum for my number of elbowsDryer manual: one maximum, no elbow figures

The code lets you size the duct by the dryer's installation instructions instead of the 35 ft method. If the manual gives a deduction per elbow, work out its maximum for your elbows and use the chart option.

Uses the 2021 International Residential Code, section M1502.4.6 (the 2015 edition has the same figures). Other editions and local amendments can set a different limit: check which code your area has adopted.

Result

Enter your duct run

Equivalent length: —
Limit: —
Elbows count as: —
Longest straight duct allowed: —

Straight duct and elbow allowances laid end to end against the limit.

Rule: 2021 IRC M1502.4.6 · Tested against 10 worked examples, all worked by hand from the rule · How this is worked out →

Questions people ask

How long can a dryer vent be?

Under the 2021 IRC the exhaust duct can be up to 35 feet, measured from where the transition duct connects to the outlet terminal outside, and every elbow shortens that by its equivalent length. The code also allows the length to be set by the dryer manufacturer’s installation instructions instead. Other code editions and local amendments can differ.

How much length does a 90-degree elbow take off a dryer vent?

In the code table a 4-inch-radius mitered 90° elbow counts as 5 feet, and a 45° counts as 2 feet 6 inches. Smooth long-radius elbows count for far less: a 10-inch-radius smooth 90° counts as 1 foot 6 inches.

Does the flexible hose behind the dryer count toward the limit?

No. The code states that the maximum length of the exhaust duct does not include the transition duct, which is the connector between the dryer and the exhaust duct system.

My dryer manual gives a longer maximum. Which do I use?

The code lists the manufacturer’s installation instructions as one of its methods, so you may use the manual’s figure. If the manual has a chart by number of elbows, use the figure for your number of elbows; the code’s elbow table applies only when the manual gives no elbow figures. A copy of the instructions has to be given to the code official at the concealment inspection, and an equivalent length over 35 feet has to be written on a permanent label or tag within 6 feet of the duct connection.

What if my run is too long either way?

The code’s third method is a dryer exhaust duct power ventilator, with the maximum length set by that product’s instructions.