Dryer Vent Length Derating Calculator
Effective airflow loss from dryer duct length and elbows.
Enter your values
ⓘ Live results
Results
- Equivalent duct length
- 30 ft
- Estimated effective airflow
- 152 CFM
- Airflow loss
- 24%
- Max recommended (smooth 4 in)
- 35 ft equivalent (typical manufacturer limit)
Quick answer
Each 90 degree elbow in a dryer duct is roughly equivalent to 5 extra feet of straight duct, and airflow drops noticeably once equivalent length passes about 25-35 feet.
Equivalent length = straight length + 5 x elbows; effective CFM derates with equivalent length
How to use the Dryer Vent Length Derating Calculator
- 1Measure the straight duct run from dryer to termination.
- 2Count 90-degree elbows and add 5 ft equivalent length each.
- 3Check the equivalent length against your dryer manufacturer's maximum, typically 25-35 ft.
Worked example
Using the values the calculator loads with:
Inputs
- Straight duct length20 ft
- 90-degree elbows2
- Dryer rated exhaust200 CFM
Results
- Equivalent duct length30 ft
- Estimated effective airflow152 CFM
- Airflow loss24%
- Max recommended (smooth 4 in)35 ft equivalent (typical manufacturer limit)
Frequently asked questions
Why do elbows matter so much?
Each elbow adds turbulence and pressure drop similar to several feet of straight pipe, choking airflow and lint clearance.
What if I exceed the limit?
Upsize the duct diameter, add an inline booster fan rated for lint, or shorten the run.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.