Dryer Vent Length Calculator

The International Residential Code (IRC) limits clothes dryer vent length to prevent lint buildup and the dryer fires that follow.
The base limit is 35 feet for most installations, but elbows count for more than their physical length, and the dryer manufacturer sometimes has a stricter spec that overrides the code.

The math:

equivalent_length = straight_run + (90°_elbows × 5) + (45°_elbows × 2.5) + flex_penalty

Standard equivalent-length deductions per IRC and most dryer manuals:

• Each 90° elbow: 5 feet equivalent
• Each 45° elbow: 2.5 feet equivalent
• Each foot of flexible pipe: counts as 1.5 feet equivalent (50% penalty)
• Booster fan: extends max from 35 ft to 60 ft (or per manufacturer)

A worked example.
A laundry room on the second floor venting through the wall, then up and out: 18 feet of straight rigid metal pipe, two 90° elbows, one 45° elbow.
Equivalent length = 18 + (2 × 5) + (1 × 2.5) = 30.5 feet.
That is under the 35 ft IRC limit and under most dryer manufacturer limits — fine.

Same install with flex pipe instead of rigid: 18 × 1.5 = 27 feet of effective flex, plus 12.5 for the elbows = 39.5 feet.
That is over the IRC limit and over most dryer specs.
Flex pipe also accumulates lint along its corrugated walls much faster than smooth rigid metal — never use flex for the long run, only for the 4-foot transition behind the dryer (and even there, semi-rigid aluminum is better than the cheap white plastic flex).

Why the limit matters.
A vent that exceeds the equivalent length restricts airflow.
The dryer compensates by running longer cycles, which wastes electricity and overheats components.
But the bigger problem is lint accumulation: lint that should be carried out the vent settles in the duct instead, where it slowly builds up.
The US Fire Administration estimates 2,900 dryer fires per year in the US, and the leading cause is failure to clean the vent.

A few practical points.
Foil flex (the silver corrugated stuff) is acceptable per IRC for short transitions but is rated for less heat than aluminum semi-rigid.
The white plastic flex sold in big-box stores is technically banned by IRC for new installations but is everywhere because it is cheap.
Long horizontal runs slope downward toward the exterior at 1/4 inch per foot to drain condensation.
And the exterior vent hood with backdraft damper should be cleaned twice a year — birds love to nest in them.