Parallel Fans CFM Calculator
Combined airflow of identical fans running in parallel.
Enter your values
Enter the number of identical fans you are working with.
Enter the cfm per fan alone in CFM.
Enter the interaction derate in %.
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.
- Ideal combined airflow
- 12,000 CFM
- Realistic combined airflow
- 10,800 CFM
- Per-fan actual share
- 5,400 CFM
Quick answer
Ideal parallel airflow is the sum of each fan's CFM, but real ducted systems typically only achieve 85-95% of that sum due to shared system resistance.
CFM_combined = n x CFM_single x derate
At a glance
| What it does | Combined airflow of identical fans running in parallel. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Number of identical fans, CFM per fan alone, Interaction derate |
| Main output | Ideal combined airflow |
| Formula | CFM_combined = n x CFM_single x derate |
| Cost | Free — no sign-up, no download |
How to use the Parallel Fans CFM Calculator
- 1Enter the number of identical fans and the CFM each produces running alone against the system.
- 2Apply a derate for shared system resistance (85-95% is typical, worse for high-resistance systems).
- 3Use the realistic combined figure for design, not the simple sum.
Inputs explained
- Number of identical fans
- Enter the number of identical fans you are working with.
- CFM per fan alone(CFM)
- Enter the cfm per fan alone in CFM.
- Interaction derate(%)
- Enter the interaction derate in %.
Worked example
Using the values the calculator loads with:
Inputs
- Number of identical fans2
- CFM per fan alone6000 CFM
- Interaction derate90 %
Results
- Ideal combined airflow12,000 CFM
- Realistic combined airflow10,800 CFM
- Per-fan actual share5,400 CFM
Frequently asked questions
Why doesn't CFM simply double with two fans?
As combined flow rises, system resistance also rises (square law), pushing the operating point back down the fan curve.
When is the derate worst?
High-resistance (steep) system curves cause the biggest shortfall versus the ideal sum.
What do I need to enter into the Parallel Fans CFM Calculator?
Just 3 values: number of identical fans, cfm per fan alone, interaction derate. 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 Parallel Fans CFM Calculator work out the answer?
Ideal parallel airflow is the sum of each fan's CFM, but real ducted systems typically only achieve 85-95% of that sum due to shared system resistance. It applies the formula CFM_combined = n x CFM_single x derate and shows the working so you can check each step by hand.
Is the Parallel Fans CFM 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.