Quick answer
A route with 20 ft of straight rigid duct, one short 45-degree elbow, and one short 90-degree elbow has an estimated equivalent length of 27.5 ft. That leaves 7.5 ft below the 35 ft general reference. The dryer manual and local requirements still control the actual installation.
How to use this dryer vent length calculator
First trace the actual exhaust route from the connection at the wall or floor to the outdoor termination. Measure every straight section along the duct centerline, including vertical rises and horizontal runs. Add those measurements for the straight rigid duct length.
Count 45-degree and 90-degree direction changes, then select the fitting profile that best matches the route. A tight mitered elbow uses more equivalent length than a broad smooth elbow. When the dryer instructions provide fitting deductions, use the custom option rather than a generic table.
Finally, choose the maximum-length source. Use the 35 ft setting only as an early planning reference. For a purchase, remodel, or real installation decision, check the exact model's installation manual and the requirements adopted where the dryer will be installed.
Dryer vent equivalent length formula
equivalent duct length = straight rigid duct length+ (number of 45-degree elbows x 45-degree equivalent length)+ (number of 90-degree elbows x 90-degree equivalent length)remaining allowance = selected maximum - equivalent duct lengthEquivalent length is not the same as physical tape-measure length. It converts the added resistance of each bend into an extra length allowance so different route layouts can be compared on one scale.
Methodology, scope, and limits
This calculator separates four questions that are often mixed together: the physical rigid-duct run, the resistance allowance for elbows, the selected equivalent-length maximum, and the short transition duct behind the appliance.
- The 35 ft option is a general 2024 IRC reference for dryer exhaust duct, not a limit that overrides the appliance manual.
- The default elbow profile uses 2.5 ft per short mitered 45-degree fitting and 5 ft per short mitered 90-degree fitting.
- The transition duct is checked separately against an 8 ft reference and is excluded from the rigid-duct equivalent total.
- The result does not evaluate duct diameter, material, crushed sections, screws, backdraft dampers, screens, termination clearance, lint condition, or real airflow.
Example calculations
Dryer vent length examples
Typical route: 20 ft straight + one 45-degree elbow + one 90-degree elbow = 27.5 ft equivalent length with the conservative short-radius profile.
Route just over the reference: 28 ft straight with the same two elbows = 35.5 ft, which is 0.5 ft over the selected 35 ft maximum.
Smoother fittings: 25 ft straight + two 45-degree and two 90-degree smooth 10-inch-radius fittings = 29.5 ft. Fitting geometry matters, so use only values that match the actual parts or the dryer instructions.
Dryer vent elbow equivalent length chart
These reference values show why an elbow should not be counted by angle alone. Radius and construction change its equivalent-length allowance. Use the dryer manufacturer's table when it differs.
| Fitting profile | 45-degree | 90-degree | 45-degree metric | 90-degree metric |
|---|---|---|---|---|
| 4 in radius, mitered | 2 ft 6 in | 5 ft | 0.76 m | 1.52 m |
| 6 in radius, smooth | 1 ft | 1 ft 9 in | 0.30 m | 0.53 m |
| 8 in radius, smooth | 1 ft | 1 ft 7 in | 0.30 m | 0.48 m |
| 10 in radius, smooth | 9 in | 1 ft 6 in | 0.23 m | 0.46 m |
What counts toward dryer vent length?
Include in the equivalent route
Include horizontal and vertical rigid exhaust sections and each direction-changing fitting from the transition connection to the outdoor outlet. Hidden offsets still count even when they are difficult to measure.
Check separately
Enter the approved transition duct behind the dryer separately. Also review the exterior hood, access for cleaning, material, support, joints, and termination. Those details affect safety and performance but cannot be reduced to one length number.
Dryer manual vs the 35 ft reference
The 35 ft reference is useful before the exact dryer model is known. It can reveal that a laundry location needs a long run or several restrictive turns. Once a model is selected, replace the generic maximum with the installation manual's maximum and use its elbow deductions if supplied.
Do not compare only the final number. A manual may define where the measurement starts, use a different fitting table, allow a listed exhaust ventilator, or impose model-specific termination conditions. Those instructions are part of the decision, not a footnote to the calculator.
When the route needs a closer review
- The result is at or above the selected maximum.
- The route disappears into a wall and the bends are unknown.
- The duct is crushed, flexible, undersized, damaged, or hard to clean.
- Drying time has increased or the dryer reports airflow warnings.
- A booster fan is being considered to make a long route work.
Vent restriction can also increase drying time and energy use. Once the route itself is understood, use the Dryer Electricity Cost Calculator to estimate operating cost from measured kWh per load rather than assuming vent length alone predicts energy use.
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FAQ
How do I calculate dryer vent length with elbows?
Add the measured straight rigid duct length to the equivalent length assigned to every 45-degree and 90-degree elbow. For the calculator's conservative 4-inch-radius mitered profile, each 45-degree elbow adds 2.5 ft and each 90-degree elbow adds 5 ft. Use the dryer manual's own values when provided.
What is the maximum dryer vent length?
A 35 ft maximum is a common general reference for rigid dryer exhaust duct, measured from the transition-duct connection to the outlet and reduced by fitting allowances. It is not universal. The dryer's installation instructions and locally adopted requirements can specify a different maximum or a different deduction table.
Does a 90-degree elbow count as 5 feet of dryer vent?
A short 4-inch-radius mitered 90-degree elbow uses a 5 ft equivalent-length reference in this calculator. Smoother, larger-radius fittings can have lower reference values. Do not assume every elbow is identical; select the closest profile or enter the value stated by the manufacturer.
Do I include the flexible hose behind the dryer?
Enter it in the transition-duct field so the calculator can check it separately. The general 35 ft calculation applies to the exhaust duct from the transition connection to the outdoor outlet, so this tool does not add the transition duct to rigid-duct equivalent length. Keep it as short as practical and use an approved transition duct.
Can I use flexible duct inside a wall?
Do not use this calculator to approve concealed flexible duct. Dryer exhaust systems have material, support, joint, termination, and concealment requirements in addition to length. Follow the appliance instructions and local requirements, and have an uncertain concealed route reviewed before closing the wall.
Can a dryer booster fan fix a vent that is too long?
No. The 2024 IRC prohibits domestic booster fans in dryer exhaust systems. A listed dryer exhaust duct power ventilator is a different device and may be used only when the appliance instructions and locally adopted requirements allow it, with installation following the ventilator manufacturer's instructions.
Why does dryer vent length matter?
Long routes and restrictive bends increase airflow resistance. Poor airflow can lengthen drying cycles, increase lint accumulation, and make troubleshooting more difficult. Equivalent length is a planning screen, while actual safety also depends on duct material, crushed sections, joints, termination, cleaning, and the dryer itself.
Is this dryer vent calculator a code-compliance check?
No. It is a route-planning calculator based on a general reference method. It cannot identify hidden bends, verify materials, inspect a termination, or determine which code edition applies at your location. Use it to find questions to resolve before installation, not as an approval document.