Calculate fan efficiency in CFM per watt, or find airflow and power draw from CFM, watts, and efficiency values for fans of any type.
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CFM Per Watt Formula
CFM per watt measures fan efficiency. It tells you how many cubic feet of air a fan moves per minute for each watt of electricity it uses.
- E = fan efficiency in CFM per watt, or CFM/W
- CFM = airflow in cubic feet per minute
- W = power draw in watts
The calculator has three functions:
- Fan Efficiency: divides airflow by power draw to find CFM/W.
- Find Airflow: multiplies efficiency by watts to estimate CFM.
- Find Watts: divides airflow by efficiency to estimate required watts.
If you enter airflow in cubic meters per minute, it is converted to CFM before the calculation. If you enter power in kilowatts, it is converted to watts before the calculation.
Typical CFM Per Watt Ranges
Use these ranges as a rough guide when interpreting the result. Actual performance depends on fan size, blade design, motor type, speed setting, and installation conditions.
| CFM/W result | General interpretation | Common comparison |
|---|---|---|
| Less than 2 | Low efficiency | Basic box fan or inefficient fan |
| 2 to 4 | Below average | Older or small bathroom fan |
| 4 to 6 | Average | Standard residential ceiling fan |
| 6 to 10 | Good | Efficient modern ceiling fan |
| 10 or more | Excellent | Top-tier efficient fan performance |
Common Unit Conversions
| Conversion | Formula |
|---|---|
| Cubic meters per minute to CFM | CFM = m³/min × 35.3147 |
| CFM to cubic meters per minute | m³/min = CFM ÷ 35.3147 |
| Kilowatts to watts | W = kW × 1000 |
| Watts to kilowatts | kW = W ÷ 1000 |
Example Calculations
Example 1: Find CFM per watt
A fan moves 300 CFM and uses 45 watts.
The fan efficiency is 6.67 CFM/W.
Example 2: Find watts from airflow and efficiency
You want 500 CFM from a fan rated at 8 CFM/W.
The estimated power draw is 62.5 watts.
FAQ
What is a good CFM per watt?
A good CFM per watt depends on the fan type, but for many residential fans, 6 to 10 CFM/W is good and 10 CFM/W or higher is excellent. A low value means the fan uses more electricity for the amount of air it moves.
Is higher CFM per watt always better?
Higher CFM per watt means better airflow efficiency, but it is not the only thing to check. You should also consider noise, room size, static pressure, installation type, and whether the airflow rating matches the speed setting you plan to use.
Why does fan efficiency change by speed setting?
Many fans use power less efficiently at some speeds than others. A lower speed may use much less power, but it also moves less air. The CFM/W value should be calculated using airflow and wattage from the same speed setting.
