System Curve Resistance Calculator
System curve coefficient and pressure at any flow.
Enter your values
Enter the known airflow in CFM.
Enter the known pressure drop in in wc.
Enter the new airflow in CFM.
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.
- 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
At a glance
| What it does | System curve coefficient and pressure at any flow. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Known airflow, Known pressure drop, New airflow |
| Main output | System coefficient (k) |
| Formula | k = P1/Q1^2; P2 = k x Q2^2 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Known airflow(CFM)
- Enter the known airflow in CFM.
- Known pressure drop(in wc)
- Enter the known pressure drop in in wc.
- New airflow(CFM)
- Enter the new airflow in CFM.
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.
What do I need to enter into the System Curve Resistance Calculator?
Just 3 values: known airflow, known pressure drop, new airflow. 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 System Curve Resistance Calculator work out the answer?
System resistance follows the square law: pressure drop is proportional to the square of airflow. It applies the formula k = P1/Q1^2; P2 = k x Q2^2 and shows the working so you can check each step by hand.
Is the System Curve Resistance 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.