Fan Curve Operating Point Calculator
Find where a fan curve meets a quadratic system curve.
Enter your values
ⓘ Live results
Results
- Operating airflow
- 200 CFM
- Operating pressure
- 2.4 in wc
- Fan curve check
- 2.4 in wc
Quick answer
The fan operates where the fan's pressure-flow curve intersects the system's resistance curve.
Fan: P = a - b*Q^2; System: P = k*Q^2; solve a - b*Q^2 = k*Q^2
How to use the Fan Curve Operating Point Calculator
- 1Fit the fan curve to P = a - b*Q^2 using two points from the manufacturer's curve.
- 2Fit the system curve to P = k*Q^2 using a known design point.
- 3The tool solves the intersection to give the true operating CFM and pressure.
Worked example
Using the values the calculator loads with:
Inputs
- Fan curve shutoff pressure (a)4.0 in wc
- Fan curve coefficient (b)0.00004 in wc per CFM^2
- System curve coefficient (k)0.00006 in wc per CFM^2
Results
- Operating airflow200 CFM
- Operating pressure2.4 in wc
- Fan curve check2.4 in wc
Frequently asked questions
Where do I get k?
Use a known operating point (design CFM and pressure) and solve k = P / Q^2, or use the system curve calculator.
Why isn't my fan hitting design CFM?
If the actual system resistance (k) is higher than designed, the true operating point shifts to lower flow — common with dirty filters or extra fittings.
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.