Fan Laws Calculator
New CFM, pressure and power from a speed change.
Enter your values
Enter the current airflow in CFM.
Enter the current speed in RPM.
Enter the new speed in RPM.
Enter the current static in in w.c..
Enter the current power in hp.
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.
- New airflow
- 1,200 CFM
- New static pressure
- 0.72 in w.c.
- New power
- 1.296 hp
- Speed ratio
- 1.2
- Power change
- 73%
Working
- 1ratio 1.2: flow x1.2, pressure x1.44, power x1.728
Quick answer
Airflow scales with speed, pressure with speed squared and power with speed cubed.
Q2 = Q1 x (N2/N1); P2 = P1 x (N2/N1)²; W2 = W1 x (N2/N1)³
At a glance
| What it does | New CFM, pressure and power from a speed change. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Current airflow, Current speed, New speed, Current static, Current power |
| Main output | New airflow |
| Formula | Q2 = Q1 x (N2/N1); P2 = P1 x (N2/N1)²; W2 = W1 x (N2/N1)³ |
| Cost | Free — no sign-up, no download |
How to use the Fan Laws Calculator
- 1Enter the current operating point — airflow, speed, static and power.
- 2Enter the new fan speed or pulley RPM.
- 3Check the new power against the motor's rating before making the change.
Inputs explained
- Current airflow(CFM)
- Enter the current airflow in CFM.
- Current speed(RPM)
- Enter the current speed in RPM.
- New speed(RPM)
- Enter the new speed in RPM.
- Current static(in w.c.)
- Enter the current static in in w.c..
- Current power(hp)
- Enter the current power in hp.
Worked example
Using the values the calculator loads with:
Inputs
- Current airflow1000 CFM
- Current speed1000 RPM
- New speed1200 RPM
- Current static0.5 in w.c.
- Current power0.75 hp
Results
- New airflow1,200 CFM
- New static pressure0.72 in w.c.
- New power1.296 hp
- Speed ratio1.2
- Power change73%
- ratio 1.2: flow x1.2, pressure x1.44, power x1.728
Frequently asked questions
Why does 20% more air cost 73% more power?
Power follows the cube of speed, so small airflow gains are expensive.
Do the laws work for slowing a fan?
Yes, and that is where they shine — dropping 10% speed cuts power by about 27%.
What do I need to enter into the Fan Laws Calculator?
Just 5 values: current airflow, current speed, new speed, current static, current power. 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 Fan Laws Calculator work out the answer?
Airflow scales with speed, pressure with speed squared and power with speed cubed. It applies the formula Q2 = Q1 x (N2/N1); P2 = P1 x (N2/N1)²; W2 = W1 x (N2/N1)³ and shows the working so you can check each step by hand.
Is the Fan Laws 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.