Dryer Vent Length Derating Calculator
Effective airflow loss from dryer duct length and elbows.
Enter your values
Enter the straight duct length in ft.
Enter the 90-degree elbows you are working with.
Enter the dryer rated exhaust in CFM.
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 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
At a glance
| What it does | Effective airflow loss from dryer duct length and elbows. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Straight duct length, 90-degree elbows, Dryer rated exhaust |
| Main output | Equivalent duct length |
| Formula | Equivalent length = straight length + 5 x elbows; effective CFM derates with equivalent length |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Straight duct length(ft)
- Enter the straight duct length in ft.
- 90-degree elbows
- Enter the 90-degree elbows you are working with.
- Dryer rated exhaust(CFM)
- Enter the dryer rated exhaust in CFM.
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.
What do I need to enter into the Dryer Vent Length Derating Calculator?
Just 3 values: straight duct length, 90-degree elbows, dryer rated exhaust. 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 Length Derating Calculator work out the 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. It applies the formula Equivalent length = straight length + 5 x elbows; effective CFM derates with equivalent length and shows the working so you can check each step by hand.
Is the Dryer Vent Length Derating 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.