Dryer Vent Length Calculator
The number everyone remembers is thirty-five feet. The number nobody remembers is that every ninety degree elbow takes five of those feet, and every forty-five takes two and a half. A run with four elbows has already spent twenty feet before a single length of duct goes in, leaving fifteen. That is why so many dryers need two cycles in houses where the duct is, on paper, well under the limit.
- Equivalent length
- 27.0 ft of 35 ft 12 ft of straight duct plus 15.0 ft of elbow penalty — 3 × 5 ft for the 90s and 0 × 2.5 ft for the 45s. Measured against IRC M1502.4.5.
- What the elbows cost you
- 15.0 ft 43% of the entire allowance, spent on 3 fittings. This is the line people miss: a run "well under 35 feet" with four 90s has only 20 ft of straight duct available.
- Straight duct still available
- 8.0 ft spare You could add 8.0 ft of straight duct, or 1 more 90° elbow, before reaching the limit.
- Rigid metal, joints taped, no screws
- 4 inch smooth wall Screws through a duct joint leave points inside that catch lint, and the blockage grows from there. Joints go together with the male end pointing downstream and are sealed on the outside with foil tape. Flexible foil duct is not permitted in the concealed portion of the run at all — its ribs trap lint by design.
- Duct diameter
- 4" Four inch is the standard and what dryers are designed around. Going smaller is never acceptable; going larger is only acceptable where the dryer manufacturer permits it, because a duct too large drops the air velocity below what carries lint out.
- Transition duct is not the vent
- Max 8 ft, listed The flexible piece between the dryer and the wall is a transition duct. It has to be listed for the purpose, it is limited in length, and it is not concealed in construction. The duct inside the wall is a different product with different rules, and using flexible foil for it is a common and serious mistake.
- Terminate outside, with a proper hood
- Never into an attic A dryer moves several litres of water per load. Vented into an attic, a crawl space or a garage, that moisture condenses on the first cold surface it meets. The termination needs a backdraft damper and no insect screen — screens are exactly the right mesh to catch lint and block completely.
- Signs it is too long
- Long cycles, hot laundry room A dryer that needs two cycles, a room that gets warm and humid while it runs, or lint appearing around the dryer body all point at duct restriction rather than a failing machine. The duct is the first thing to check and the last thing anyone checks.
Equivalent length, not measured length
A fitting resists airflow far more than the same length of straight duct, so the code expresses the limit in equivalent length: measured straight duct plus a penalty for every change of direction.
Under the IRC the maximum is thirty-five feet of equivalent length, with five feet subtracted for each ninety degree elbow and two and a half for each forty-five. A manufacturer may permit more, and where the instructions give a longer allowance those instructions govern.
A typical installation has an elbow leaving the dryer, one turning up or down inside the wall, and one or two more getting through the framing to the outside. Four elbows is unremarkable, and it takes twenty feet.
Where geometry allows, two forty-five degree elbows do the same job as one ninety for the same total turn and cost five feet instead of five — no saving in that case. The real saving is avoiding turns altogether, and a duct that leaves through the nearest exterior wall beats one that travels across the house to a tidier termination.
What a restricted duct actually does
A dryer removes several litres of water from a load of laundry and it moves that water out as vapour in the exhaust stream. If the duct restricts the flow, the moisture does not leave.
The immediate symptom is longer cycles, because the machine is recirculating damp air. The secondary symptom is a laundry room that gets warm and humid while the dryer runs, which is heat and moisture that should have gone outside.
The dangerous consequence is lint. Lint stays airborne only while the air is moving fast enough to carry it; below roughly twelve hundred feet per minute it settles. A restricted duct is a slow duct, so it accumulates the lint it should be carrying away, and the accumulation restricts it further.
That feedback loop is why dryer ducts are a recognised fire cause. The heating element is on the supply side of a duct progressively filling with dry, fine, flammable material.
The installation details that cause blockages
No screws. A sheet metal screw through a duct joint leaves half an inch of point inside the airstream, and lint catches on it immediately. Joints assemble with the male end pointing in the direction of flow and are sealed on the outside with foil tape.
Rigid smooth-wall metal for the concealed run. Flexible foil duct has ribs that trap lint by design, and it is not permitted where the duct is concealed in construction. The short flexible piece behind the dryer is a transition duct, a separate listed product with its own length limit, and it is never concealed.
No insect screen at the termination. The mesh size that keeps insects out is exactly the mesh size that catches lint, and a screened hood blocks completely within a year. A backdraft damper is the correct termination and it excludes what needs excluding.
And it terminates outside. Not into an attic, not into a crawl space, not into a garage. Several litres of water per load condensing on the first cold surface it finds is a mould problem being manufactured on a schedule.
What this is based on
- IRC M1502.4.5 — maximum dryer exhaust duct length and elbow equivalent lengths
- IRC M1502.4 — duct material, joint construction and termination requirements
A length check against the IRC. Where the dryer manufacturer specifies a different maximum length or duct diameter, those instructions govern. It does not cover booster fans, common exhaust systems in multi-family buildings, or make-up air requirements for very high capacity machines.
Frequently asked questions
How long can a dryer vent be?
Thirty-five feet of equivalent length under the IRC, less five feet for every 90 degree elbow and two and a half for every 45. A manufacturer may permit more, and where the instructions allow a longer run those instructions govern.
How much does an elbow subtract?
Five feet for a 90 degree elbow, two and a half for a 45. Four elbows — which is a very ordinary installation — take twenty feet of the thirty-five, leaving fifteen for actual duct.
Can I use flexible foil duct?
Not for the concealed run. Its ribs trap lint by design. The short flexible piece behind the dryer is a transition duct, which is a separate listed product with a length limit, and it must not be concealed in construction.
Why should I not use screws on duct joints?
Every screw leaves a point inside the airstream that catches lint, and the blockage grows from there. Joints go together male-end-downstream and are sealed on the outside with foil tape.
Why does my dryer take two cycles?
Usually duct restriction rather than a failing machine. A dryer removes several litres of water per load as vapour, and a restricted duct cannot carry it out — so the machine recirculates damp air. Check the duct before the appliance.