Parallel Fans CFM Calculator
Combined airflow of identical fans running in parallel.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.