Dryer Vent Airflow Calculator
Vent length allowance and airflow check.
Enter your values
ⓘ Live results
Results
- Equivalent length
- 30.5 ft
- Allowance
- 35 ft
- Verdict
- Within allowance
- Headroom left
- 4.5 ft
- Typical dryer airflow
- ~150-200 CFM
Quick answer
Most codes allow 35 feet of 4-inch dryer duct, minus 5 feet per 90-degree elbow and 2.5 feet per 45.
equivalent length = straight + 5 x 90s + 2.5 x 45s
How to use the Dryer Vent Airflow Calculator
- 1Measure the straight duct from the dryer to the exterior hood.
- 2Count the 90 and 45 degree elbows in the run.
- 3Use smooth rigid metal duct — flexible foil eats the allowance fast.
Worked example
Using the values the calculator loads with:
Inputs
- Straight duct length18 ft
- Number of 90° elbows2
- Number of 45° elbows1
- Allowance35 ft
Results
- Equivalent length30.5 ft
- Allowance35 ft
- VerdictWithin allowance
- Headroom left4.5 ft
- Typical dryer airflow~150-200 CFM
Frequently asked questions
Does the hood count?
Yes — a typical exterior hood costs several equivalent feet; check the maker's table.
Why does it matter?
Restricted dryer vents lengthen cycles and are a leading cause of lint fires.
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.