Fans in Series vs Parallel Calculator — Combined Fan Curve and Real Airflow
Do two fans in parallel double the airflow? It depends on the system resistance. Enter one fan's curve, the number of fans and your system pressure; the calculator plots the combined curve — parallel adds airflow at equal pressure, series adds pressure at equal airflow — and shows how much more air you actually get in your system.
No signup · Works on mobile · Formulas shown below · Updated 2026-11-02
Multiple-Fan Combination
Parallel: Qtotal(P) = N × Q₁(P). Series: Ptotal(Q) = N × P₁(Q). The operating point is where the combined curve meets the system curve P = P₀ + K·Q².
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
Combined fan curves in parallel and in series
Parallel (airflow adds at equal pressure) Qtotal(P) = Q₁(P) + Q₂(P) + … = N × Q₁(P) Series (pressure adds at equal airflow) Ptotal(Q) = P₁(Q) + P₂(Q) + … = N × P₁(Q) System curve P = P₀ + K × Q² Operating point intersection of the combined curve and the system curve // Ideal addition; real installations lose some performance to interference, inlet/outlet conditions and swirl between series stages
The key is that the system curve is a parabola. Parallel fans stretch the curve to the right, which helps most on a flat (low-resistance) system. Series fans stretch it upwards, which helps most on a steep (high-resistance) system. “Two fans in parallel double the CFM” is only true when the system resistance is close to zero.
Fans in series vs parallel: when to use which
Situation
Recommended
Why
High airflow, low resistance (ventilation, cooling, fan walls)
Parallel
Airflow rises almost in proportion to the number of fans; allows N+1 redundancy
Long ducts, multi-stage filtration, high resistance
Series, or one higher-pressure fan
Most of the extra parallel airflow is absorbed by the resistance
Must keep running during a fan failure or maintenance
Several fans in parallel
The remaining fans keep running (backdraft dampers required)
Wide turndown range
Parallel with fan staging or EC speed control
At part load, switch fans off or slow them all down for better efficiency
Worked example: why two fans in parallel added only 10 %
Given: one EC plug fan with catalog points 0 CFM / 2.41 in. w.g., 880 / 2.25, 1,770 / 1.81, 2,650 / 1.08 and 3,240 CFM / 0.40 in. w.g.; the system needs 2.09 in. w.g. at 1,770 CFM (a high-resistance system).
One fan on this system: about 1,670 CFM at 1.86 in. w.g.
Two fans in parallel: about 1,830 CFM at 2.23 in. w.g. — only about 10 % more air
Why: the extra airflow raises system resistance with the square of airflow, pushing both fans up to the left end of their curve, close to shut-off pressure
The same two fans in series give about 2,130 CFM (+28 %); a single higher-pressure fan is another option. Load the preset “high-resistance system” and switch the arrangement to compare.
Frequently Asked Questions
Do two fans in parallel double the airflow?
Only when system resistance is very low. With more airflow, duct resistance rises with the square of airflow, so both fans end up running at higher pressure and lower airflow each. On a high-resistance system, two fans in parallel often add only 10–40 % airflow.
Do two fans in series double the pressure?
At the same airflow the combined pressure is double in theory, but the operating point moves to a higher airflow, so the actual static pressure gain is less than double. Swirl from the first fan and connection losses reduce it further.
What should I watch out for with fans in parallel?
Use the same model at the same speed, with a curve that falls steadily through the operating range; fit a backdraft damper on each fan; and leave enough inlet and outlet space so the fans do not disturb each other. With mismatched fans, the weaker one can be pushed into stall or reverse flow.
Is a fan wall (fan array) a parallel arrangement?
Yes. A fan wall or fan array places several EC plug fans in parallel across one cross-section. Benefits include N+1 redundancy, lower noise and flexible control by number of fans and speed, which is why fan arrays are common in air handlers and data centers.
What is the difference between fans in series and a contra-rotating fan?
A contra-rotating fan is a series arrangement of two impellers turning in opposite directions; the second stage recovers the swirl from the first as pressure. It is more efficient than simply stacking two axial fans turning the same way, and is used for tunnels and mines that need high pressure and airflow.
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