System Curve Resistance Calculator
System curve coefficient and pressure at any flow.
Enter your values
ⓘ Live results
Results
- System coefficient (k)
- 6e-8
- Pressure at new flow
- 2.722 in wc
- Flow ratio
- 1.167
- Pressure ratio
- 1.361
Quick answer
System resistance follows the square law: pressure drop is proportional to the square of airflow.
k = P1/Q1^2; P2 = k x Q2^2
How to use the System Curve Resistance Calculator
- 1Enter a known measured airflow and its corresponding pressure drop.
- 2Enter the new airflow you want to predict pressure for.
- 3The tool applies the fan/duct square law to give the new pressure drop.
Worked example
Using the values the calculator loads with:
Inputs
- Known airflow6000 CFM
- Known pressure drop2.0 in wc
- New airflow7000 CFM
Results
- System coefficient (k)6e-8
- Pressure at new flow2.722 in wc
- Flow ratio1.167
- Pressure ratio1.361
Frequently asked questions
Does this work for VAV systems?
Only approximately, since damper positions change the effective resistance; best for constant-configuration ductwork.
Why square law?
Turbulent duct friction losses scale with velocity squared, which scales with CFM squared for fixed duct area.
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.