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Dryer Vent Length Calculator
A dryer duct has a fixed allowance that every elbow spends, and a termination the appliance instructions decide before the code does. Both are checkable in ninety seconds while the wall is open, and effectively uncheckable afterwards.
Elbows are the expensive part
The maximum developed length of a clothes dryer exhaust duct is 35 feet, measured from the transition-duct connection to the outlet terminal, and each fitting is deducted from that allowance as an equivalent length — 5 feet for a 4-inch 90-degree elbow and 2 feet 6 inches for a 45.
Where the dryer's own instructions are more restrictive, they govern. An unanswered item here is reported as unanswered rather than read the favourable way — a route that has not been measured is not a route that fits.
1. The length budget
Developed length is the path the duct actually takes, measured from the connection to the transition duct at the dryer to the outlet terminal. It does not include the flexible transition duct behind the appliance.
2. Where the duct ends
The termination location is governed first by the dryer's installation instructions. The code default applies only where those instructions do not specify one.
Is the termination fitted with a backdraft damper?
Is there a screen or insect mesh at the termination?
Screens are not permitted at a dryer duct termination. Lint blinds them, and a blinded screen closes the duct.
3. The duct itself
Is the duct rigid metal with a smooth interior, 4 inches nominal, with no fasteners protruding into the duct?
Not less than 0.016 inch thick, joints made in the direction of airflow, supported at intervals not exceeding 12 feet.
Is the transition duct a single listed length no greater than 8 feet, left unconcealed?
The flexible connector behind the appliance is listed and labelled to UL 2158A, a single length not greater than 8 feet, and not concealed within construction.
Start with the route
Two things decide whether a dryer duct complies: where it ends, and how much of the 35-foot allowance the route has spent. Enter the straight run and the elbow count, then work through the termination.
Informational use only, please verify before you rely on it
This reproduces the structure of IRC Section M1502. The maximum-length provision and its fitting table are numbered M1502.4.5.1 in the 2015 and 2018 editions and M1502.4.6.1 in the 2021 and 2024 editions; the substantive figures are materially unchanged. Only 4-inch nominal duct is covered, because that is the diameter the residential provision specifies — upsized duct is an engineered case with its own table values. This is not a determination. Jurisdictions adopt different editions and amend mechanical provisions, and the dryer manufacturer's installation instructions govern the termination location and may set a shorter maximum length. Confirm with the building department that will issue the permit.
This tool is provided for general informational and educational purposes only. Its output is an illustrative estimate generated from the values you enter and from general assumptions that will not match every deal, market, lender, or homeowner. It is not tax, legal, accounting, financial, or professional advice, and it is not a quote, an offer, a credit decision, or a guarantee of pricing, approval, timing, savings, or eligibility.
You are solely responsible for independently confirming all information presented here including any figures, rates, fees, margins, timelines, tax treatment, and federal, state, local, or utility incentives, with the applicable lender, authority having jurisdiction, and your own qualified tax, legal, and financial advisors before acting on it, relying on it, or presenting it to a homeowner or any third party. Incentive programs, lender terms, and permitting requirements change frequently and vary by jurisdiction.
Seamless Home is not a tax advisor, law firm, lender, or licensed installing contractor, and makes no representation or warranty as to the accuracy, completeness, or currency of the information produced by this tool. To the fullest extent permitted by law, Seamless Home accepts no liability for any decision made or action taken in reliance on it.
Why the route is a design decision
The duct route is usually fixed by framing and cabinet layout long before anyone with a mechanical background looks at it, and whoever connects it inherits the elbow count the framing left behind. Where a dryer vent can and cannot terminate sets out the termination rules in full, including the reason a ventilated soffit is the one place a termination most conclusively does not belong.
It is the same shape of problem as every other concealed line on a remodel. Where an HVAC condensate drain can discharge covers a small-bore path above a finished ceiling whose failure is expensive and whose compliance is unverifiable once closed, and the condensate overflow checker works the same way this one does.
Which edition and which local amendments govern is the authority having jurisdiction's answer, settled at permit rather than argued at inspection. For the work around a laundry relocation or a basement finish, see remodeling and home improvements.
Frequently Asked Questions
What does the 35-foot limit measure?+
Developed length — the route the rigid duct actually takes through the building, measured from the connection to the transition duct at the dryer to the outlet terminal. It is not a straight-line distance and it does not include the flexible transition duct behind the appliance. Fittings are then deducted from that allowance as equivalent lengths, because a bend costs airflow that straight duct does not.
How much does each elbow cost?+
For 4-inch duct the model table assigns 5 feet of equivalent length to a 90-degree elbow and 2 feet 6 inches to a 45-degree elbow. The arithmetic is what makes laundry-room location a design decision: four 90-degree elbows spend 20 of the 35 feet before any straight pipe is counted, which is why a laundry room in the middle of a floor plan is a different proposition from one on an exterior wall.
Why does the tool ask for the manufacturer's maximum?+
Because where the appliance is more restrictive than the code default, the appliance governs. Dryer manufacturers publish their own maximum duct lengths and 25 feet is common. A run that satisfies the 35-foot code figure but exceeds the dryer's own stated maximum is not a compliant installation, because the code requires the appliance to be installed in accordance with its listing — and it is a warranty problem as well as a code one.
Why is a screen at the termination treated as a defect?+
Because the code states that screens shall not be installed at the duct termination, and because the failure mode is predictable. Lint accumulates on any screen, and a blinded screen converts the duct into a closed pipe. The required component is a backdraft damper instead — a flap that opens on airflow and closes by gravity — which does the job an insect screen was meant to do without giving lint anything to build on. Retrofitting mesh over a dryer cap is a common well-meant modification that creates the exact restriction the section exists to prevent.
Why is a ventilated soffit treated the same as an attic?+
Because it is one. A ventilated soffit is an intake for the attic, so a soffit termination puts humid, lint-laden air directly into the path that draws air up under the roof deck. The code closes the gap explicitly by naming openings in ventilated soffits among the openings a termination must be held clear of. It looks like the outdoors from the ground, which is exactly why it keeps being used.
Why does the tool only handle 4-inch duct?+
Because 4 inches nominal is the diameter the residential provision specifies, and it is the case this tool can reproduce accurately. Larger-diameter duct has its own equivalent-length values and is an engineered case that should be confirmed against the adopted code text and the appliance instructions rather than estimated here. Reporting a figure this tool cannot verify would be worse than declining to.
Is this a determination?+
No. It reproduces the structure of IRC Section M1502 and is a preparation aid, not a compliance ruling. Jurisdictions adopt different editions and amend mechanical provisions, the termination location is governed first by the dryer manufacturer's installation instructions, and the numbering of the length provision differs between editions — M1502.4.5.1 in the 2015 and 2018 editions, M1502.4.6.1 in the 2021 and 2024 editions. Confirm against the edition your building department has adopted.
Related resources
Where a Dryer Vent Can and Cannot Terminate
The termination rules, the length budget and the tag that survives the drywall.
Learn more →Remodeling and Home Improvements
Where laundry relocations and basement finishes get coordinated.
Learn more →HVAC Condensate Overflow Protection Checker
The same question on the other concealed line above a finished ceiling.
Learn more →Does Your Furnace or Water Heater Have Enough Combustion Air?
The air an appliance closet needs, and what a replacement changes.
Learn more →Settle the duct route before the wall closes
Seamless Home owns the design and permit layer on the work it runs, which is where a laundry duct route stops being the framer's problem and becomes a documented one. Coverage is confirmed per service area rather than promised as blanket availability.
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