Fan Efficiency Calculator
Total fan efficiency from airflow, pressure and power.
Enter your values
Enter the airflow in CFM.
Enter the total pressure in in wc.
Enter the input power in W.
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.
- Fan efficiency
- 40.2%
- Air power
- 0.378 HP
- Specific fan power
- 0.74 W per L/s
- Watts per CFM
- 0.35
- Rating
- Poor — check duct restriction
Quick answer
Fan efficiency is air power divided by shaft or input power; good HVAC fans land between 55% and 75%.
eff = CFM x in wc / (6356 x HP)
At a glance
| What it does | Total fan efficiency from airflow, pressure and power. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Total pressure, Input power |
| Main output | Fan efficiency |
| Formula | eff = CFM x in wc / (6356 x HP) |
| Cost | Free — no sign-up, no download |
How to use the Fan Efficiency Calculator
- 1Measure airflow and total pressure rise across the fan.
- 2Read input watts at the plug or from the VFD.
- 3Compare watts per CFM — under 0.5 is good for a residential ECM blower.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Total pressure(in wc)
- Enter the total pressure in in wc.
- Input power(W)
- Enter the input power in W.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow2000 CFM
- Total pressure1.2 in wc
- Input power700 W
Results
- Fan efficiency40.2%
- Air power0.378 HP
- Specific fan power0.74 W per L/s
- Watts per CFM0.35
- RatingPoor — check duct restriction
Frequently asked questions
Why is my efficiency low?
Usually excessive system static, a dirty wheel, or a fan operating far off its best efficiency point.
What is specific fan power?
Watts per litre per second — European standards often cap it at 1.6 W/(L/s) for balanced systems.
What do I need to enter into the Fan Efficiency Calculator?
Just 3 values: airflow, total pressure, input 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 Efficiency Calculator work out the answer?
Fan efficiency is air power divided by shaft or input power; good HVAC fans land between 55% and 75%. It applies the formula eff = CFM x in wc / (6356 x HP) and shows the working so you can check each step by hand.
Is the Fan Efficiency 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.