Dryer Vent Length Efficiency Calculator

Your dryer works by moving warm air through the drum, picking up moisture, and pushing that humid air out through the dryer vent (dryer duct). When the exhaust path is short and smooth, the blower can maintain good airflow. As the vent run gets longer—or as you add elbows—the duct offers more resistance. The result is usually longer drying times, higher energy use, and faster lint buildup. This calculator estimates dryer vent effective length and a simplified vent-length efficiency score so you can quickly see whether your setup is likely to be “in range” of common guidance.

What this calculator does

• Effective length: converts bends into an equivalent amount of straight duct and adds it to your measured straight run.
• Efficiency estimate: compares your effective length to a common reference limit (35 ft) and expresses the result as a percentage.
• Interpretation: helps you decide whether to leave things as-is, monitor/clean more often, or consider improvements (shorter route, fewer elbows, better duct material, etc.).

Inputs

• Vent length (ft): the approximate straight-line duct length from the dryer outlet to the exterior termination (include segments you can measure). If you only have a rough estimate, err slightly high.
• Number of 90° bends: count the right-angle elbows (or similarly sharp turns). Two 45° bends are often roughly comparable to one 90° bend in rule-of-thumb planning, but this calculator specifically uses the number of 90° bends you enter.

Formulas (effective length and efficiency)

The calculator uses a common planning rule that treats each 90° elbow as adding about 5 ft of equivalent straight duct due to added friction and turbulence.

Effective length

L = l + 5b

Efficiency model (relative to a 35 ft reference)

η = 1 − L/35

To present the result as a percentage:

Efficiency (%) = 100 × η

Here is the same efficiency formula in MathML:

How to interpret the results

Effective length is the main “planning” number: if it is well under the reference limit, airflow is more likely to be adequate (assuming a reasonably clean, properly installed duct). If effective length is near or above the reference, small real‑world factors—lint, crushed flex duct, a restrictive hood, or extra fittings—can push performance into “slow drying” territory.

Efficiency (%) is a simplified indicator that scales down linearly as effective length increases. It is best used as a relative score (shorter and straighter is better), not as a precise prediction of run time or energy cost.

Quick guidance table

Worked example

Suppose you measure about 22 ft of duct from the dryer to the exterior wall and you count 3 90° elbows.

1. Effective length: L = 22 + 5×3 = 37 ft
2. Efficiency: η = 1 − 37/35 ≈ −0.057
3. Efficiency (%): ≈ −5.7%

An efficiency below 0% indicates the effective length exceeds the 35 ft reference. In the real world, that doesn’t mean the dryer “won’t work,” but it does mean you’re in the zone where small restrictions (lint, flex duct sagging, tight elbow radius, a flap hood that sticks) can have an outsized impact. If drying takes multiple cycles, this is a strong hint to inspect/clean and consider rerouting.

Why bends and duct details matter

Longer vents reduce air velocity, which can extend drying time. Elbows are often worse than the same added length of straight duct because the airflow must change direction, creating turbulence and localized pressure losses. That turbulence can also encourage lint to deposit—especially if the duct is corrugated flexible material, has sags, or has joints that catch lint.

Assumptions & limitations (important)

• Rule-of-thumb elbow equivalent: the “5 ft per 90° bend” factor is a common approximation. Actual equivalent length depends on elbow radius, duct diameter, and airflow rate.
• Reference limit varies: the 35 ft guideline is commonly cited in residential guidance, but allowable maximum length can differ by dryer manufacturer, model, and local code, and may be reduced by each fitting.
• Linear efficiency is simplified: real pressure loss vs. length is not perfectly linear across all installations. The percentage is meant as a comparative indicator, not a guaranteed performance prediction.
• Does not model key real‑world restrictions: duct diameter, duct material (rigid vs. flexible), crushed sections, sags, number of joints, transitions, lint screens, termination hood type, and wind effects can change airflow substantially.
• Cleanliness matters: lint accumulation can dominate performance even when the vent is “within length.” If cycles are getting longer, inspect/clean the lint screen path and venting system.
• Safety note: restricted vents are associated with increased lint buildup and overheating risk. If you suspect a blockage or overheating, stop using the dryer and seek qualified help.

What to do if your effective length is high

• Reduce the route: shorten the run or move the dryer closer to an exterior wall if practical.
• Reduce elbows: gentler routing and fewer sharp turns can improve airflow.
• Use smoother ducting: rigid or semi‑rigid metal ducts typically perform better than long runs of flexible corrugated duct.
• Confirm the exterior hood: ensure the damper opens freely and isn’t blocked by lint, paint, or a pest screen that traps lint.
• Follow the manufacturer: check your dryer’s installation manual for maximum vent length rules for your exact model.

Notes on sources

This page uses widely repeated residential planning guidance for dryer exhaust length (often summarized as a base maximum length with deductions or equivalents for fittings). Always defer to your dryer manufacturer’s installation instructions and applicable local building code requirements for final decisions.