Fan Affinity Speed Calculator
CFM, pressure and power from a fan speed change.
Enter your values
Enter the current cfm in CFM.
Enter the current static pressure in in wc.
Enter the current brake hp in hp.
Enter the current rpm in rpm.
Enter the new rpm in rpm.
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
- 9,333 CFM
- New static pressure
- 2.042 in wc
- New brake horsepower
- 7.94 hp
- Speed ratio
- 1.167
- Power change
- 58.8%
Quick answer
Airflow scales linearly with speed, pressure with the square of speed, and power with the cube of speed.
CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)^2; BHP2 = BHP1 x (N2/N1)^3
At a glance
| What it does | CFM, pressure and power from a fan speed change. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Current CFM, Current static pressure, Current brake HP, Current RPM, New RPM |
| Main output | New airflow |
| Formula | CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)^2; BHP2 = BHP1 x (N2/N1)^3 |
| Cost | Free — no sign-up, no download |
How to use the Fan Affinity Speed Calculator
- 1Enter the fan's current CFM, static pressure and brake horsepower at its current speed.
- 2Enter the current and new RPM.
- 3Read the new CFM, pressure and power computed from the affinity laws.
Inputs explained
- Current CFM(CFM)
- Enter the current cfm in CFM.
- Current static pressure(in wc)
- Enter the current static pressure in in wc.
- Current brake HP(hp)
- Enter the current brake hp in hp.
- Current RPM(rpm)
- Enter the current rpm in rpm.
- New RPM(rpm)
- Enter the new rpm in rpm.
Worked example
Using the values the calculator loads with:
Inputs
- Current CFM8000 CFM
- Current static pressure1.5 in wc
- Current brake HP5 hp
- Current RPM1200 rpm
- New RPM1400 rpm
Results
- New airflow9,333 CFM
- New static pressure2.042 in wc
- New brake horsepower7.94 hp
- Speed ratio1.167
- Power change58.8%
Frequently asked questions
Why does power rise so fast?
Power follows the cube law, so a modest speed increase produces a much larger power increase.
Do the fan laws always apply?
They assume the system curve and air density stay the same and the fan operates in its normal range.
What do I need to enter into the Fan Affinity Speed Calculator?
Just 5 values: current cfm, current static pressure, current brake hp, current rpm, new rpm. 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 Affinity Speed Calculator work out the answer?
Airflow scales linearly with speed, pressure with the square of speed, and power with the cube of speed. It applies the formula CFM2 = CFM1 x (N2/N1); SP2 = SP1 x (N2/N1)^2; BHP2 = BHP1 x (N2/N1)^3 and shows the working so you can check each step by hand.
Is the Fan Affinity Speed 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.