Fan RPM Change Calculator
Required RPM to hit a target CFM.
Enter your values
Enter the current cfm in CFM.
Enter the current rpm in rpm.
Enter the target cfm 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.
- Required RPM
- 1,160 rpm
- RPM change
- 160 rpm
- Speed ratio
- 1.16
- Expected pressure ratio
- 1.346
- Expected power ratio
- 1.561
Quick answer
Required RPM is the current RPM multiplied by the ratio of target CFM to current CFM.
N2 = N1 x (CFM2/CFM1)
At a glance
| What it does | Required RPM to hit a target CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Current CFM, Current RPM, Target CFM |
| Main output | Required RPM |
| Formula | N2 = N1 x (CFM2/CFM1) |
| Cost | Free — no sign-up, no download |
How to use the Fan RPM Change Calculator
- 1Enter your current CFM and RPM from a field measurement or fan curve.
- 2Enter the target CFM you need.
- 3Read the RPM to dial in on the drive or VFD, plus the expected pressure and power change.
Inputs explained
- Current CFM(CFM)
- Enter the current cfm in CFM.
- Current RPM(rpm)
- Enter the current rpm in rpm.
- Target CFM(CFM)
- Enter the target cfm in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Current CFM5000 CFM
- Current RPM1000 rpm
- Target CFM5800 CFM
Results
- Required RPM1,160 rpm
- RPM change160 rpm
- Speed ratio1.16
- Expected pressure ratio1.346
- Expected power ratio1.561
Frequently asked questions
Can I just set the VFD to this frequency?
Yes if driven by a VFD at fixed motor poles — RPM is proportional to frequency.
What if I have a belt drive?
Use this RPM as the target fan shaft speed, then resize the sheaves accordingly.
What do I need to enter into the Fan RPM Change Calculator?
Just 3 values: current cfm, current rpm, target cfm. 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 RPM Change Calculator work out the answer?
Required RPM is the current RPM multiplied by the ratio of target CFM to current CFM. It applies the formula N2 = N1 x (CFM2/CFM1) and shows the working so you can check each step by hand.
Is the Fan RPM Change 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.