Fan Affinity Diameter Calculator
CFM, pressure and power from a fan wheel diameter 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 wheel diameter in in.
Enter the new wheel diameter in in.
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
- 8,230 CFM
- New static pressure
- 1.481 in wc
- New brake horsepower
- 6.77 hp
- Diameter ratio
- 1.111
Quick answer
At constant RPM, airflow scales with the cube of wheel diameter, pressure with the square, and power with the fifth power.
CFM2 = CFM1 x (D2/D1)^3; SP2 = SP1 x (D2/D1)^2; BHP2 = BHP1 x (D2/D1)^5
At a glance
| What it does | CFM, pressure and power from a fan wheel diameter change. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Current CFM, Current static pressure, Current brake HP, Current wheel diameter, New wheel diameter |
| Main output | New airflow |
| Formula | CFM2 = CFM1 x (D2/D1)^3; SP2 = SP1 x (D2/D1)^2; BHP2 = BHP1 x (D2/D1)^5 |
| Cost | Free — no sign-up, no download |
How to use the Fan Affinity Diameter Calculator
- 1Enter the fan's current CFM, pressure and horsepower with its current wheel diameter.
- 2Enter the new wheel diameter.
- 3Read predicted CFM, pressure and power at the same RPM.
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 wheel diameter(in)
- Enter the current wheel diameter in in.
- New wheel diameter(in)
- Enter the new wheel diameter in in.
Worked example
Using the values the calculator loads with:
Inputs
- Current CFM6000 CFM
- Current static pressure1.2 in wc
- Current brake HP4 hp
- Current wheel diameter18 in
- New wheel diameter20 in
Results
- New airflow8,230 CFM
- New static pressure1.481 in wc
- New brake horsepower6.77 hp
- Diameter ratio1.111
Frequently asked questions
Does the motor need upsizing?
Check the new brake horsepower against the motor nameplate — the fifth-power rule can exceed it quickly.
Is this valid for any fan family?
Only within the same fan family and similar blade geometry.
What do I need to enter into the Fan Affinity Diameter Calculator?
Just 5 values: current cfm, current static pressure, current brake hp, current wheel diameter, new wheel diameter. 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 Diameter Calculator work out the answer?
At constant RPM, airflow scales with the cube of wheel diameter, pressure with the square, and power with the fifth power. It applies the formula CFM2 = CFM1 x (D2/D1)^3; SP2 = SP1 x (D2/D1)^2; BHP2 = BHP1 x (D2/D1)^5 and shows the working so you can check each step by hand.
Is the Fan Affinity Diameter 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.