Dryer Vent Airflow Calculator
Vent length allowance and airflow check.
Enter your values
Enter the straight duct length in ft.
Enter the number of 90° elbows you are working with.
Enter the number of 45° elbows you are working with.
Enter the allowance in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Vent length allowance and airflow check. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Straight duct length, Number of 90° elbows, Number of 45° elbows, Allowance |
| Main output | Equivalent length |
| Formula | equivalent length = straight + 5 x 90s + 2.5 x 45s |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Straight duct length(ft)
- Enter the straight duct length in ft.
- Number of 90° elbows
- Enter the number of 90° elbows you are working with.
- Number of 45° elbows
- Enter the number of 45° elbows you are working with.
- Allowance(ft)
- Enter the allowance in ft.
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.
What do I need to enter into the Dryer Vent Airflow Calculator?
Just 4 values: straight duct length, number of 90° elbows, number of 45° elbows, allowance. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Dryer Vent Airflow Calculator work out the answer?
Most codes allow 35 feet of 4-inch dryer duct, minus 5 feet per 90-degree elbow and 2.5 feet per 45. It applies the formula equivalent length = straight + 5 x 90s + 2.5 x 45s and shows the working so you can check each step by hand.
Is the Dryer Vent Airflow Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.