Fan CFM from RPM Calculator
Apply fan law one to 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 wc.
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
- 1,250 CFM
- New static pressure
- 0.781 in wc
- New power
- 0.977 HP
- Speed ratio
- 1.25
- Power change
- 95.3%
Quick answer
Airflow scales directly with fan speed, pressure with the square of speed and power with the cube.
CFM2 = CFM1 x (RPM2 / RPM1)
At a glance
| What it does | Apply fan law one to a speed change. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Current airflow, Current speed, New speed, Current static, Current power |
| Main output | New airflow |
| Formula | CFM2 = CFM1 x (RPM2 / RPM1) |
| Cost | Free — no sign-up, no download |
How to use the Fan CFM from RPM Calculator
- 1Enter the airflow, static pressure and power at the current speed.
- 2Enter the new RPM you plan to run.
- 3Check the motor can handle the cubed power increase before speeding a fan up.
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 wc)
- Enter the current static in in wc.
- Current power(HP)
- Enter the current power in HP.
Worked example
Using the values the calculator loads with:
Inputs
- Current airflow1000 CFM
- Current speed800 RPM
- New speed1000 RPM
- Current static0.5 in wc
- Current power0.5 HP
Results
- New airflow1,250 CFM
- New static pressure0.781 in wc
- New power0.977 HP
- Speed ratio1.25
- Power change95.3%
Frequently asked questions
Why does slowing down save so much?
Power follows the cube of speed — a 20% speed cut halves the power draw.
Do the laws always hold?
They assume the same fan, air density and system curve; big density changes need correction.
What do I need to enter into the Fan CFM from RPM 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 CFM from RPM Calculator work out the answer?
Airflow scales directly with fan speed, pressure with the square of speed and power with the cube. It applies the formula CFM2 = CFM1 x (RPM2 / RPM1) and shows the working so you can check each step by hand.
Is the Fan CFM from RPM 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.