Calculate the power required to move air through a fan or duct system. Enter the air flow rate, static pressure in inches of water gauge, and fan efficiency to find the drive power in kilowatts.
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CFM to kW Formula
The following formula is used to convert CFM to kW:
kW = 0.0001175 * (CFM * P) / n
Variables:
- kW is the power in kilowatts
- CFM is the air flow in cubic feet per minute
- P is the static pressure in inches of water gauge (in. w.g.)
- n is the fan efficiency as a decimal (0 to 1)
The constant 0.0001175 is a unit conversion factor derived from physics: 1 CFM equals 0.000471947 m/s and 1 in. w.g. equals 249.089 Pa. Their product is 0.1175 W of theoretical air power, or 0.0001175 kW. Dividing by efficiency n converts from shaft input power to this theoretical air power output.
| Air Flow (CFM) | Power (kW) |
|---|---|
| 100 | 0.036 |
| 150 | 0.054 |
| 200 | 0.072 |
| 250 | 0.090 |
| 300 | 0.108 |
| 400 | 0.145 |
| 500 | 0.181 |
| 600 | 0.217 |
| 750 | 0.271 |
| 900 | 0.325 |
| 1000 | 0.362 |
| 1200 | 0.434 |
| 1500 | 0.542 |
| 2000 | 0.723 |
| 2500 | 0.904 |
| 3000 | 1.085 |
| 3500 | 1.265 |
| 4000 | 1.446 |
| 5000 | 1.808 |
| 6000 | 2.169 |
| Rounded to 3 decimals. Formula: kW = 0.0001175 x CFM x ΔP / η. Assumes ΔP = 2.0 in. w.g. and efficiency η = 0.65. | |
Fan Power vs. Air Compressor Power
This formula applies to fans and blowers where static pressure is measured in inches of water gauge. Air compressors operate at far higher pressures measured in PSI. Since 1 PSI equals 27.68 in. w.g., applying the fan formula to a compressor system would understate actual power by roughly 28x.
| System Type | Pressure Unit | Power Formula | Typical Operating Pressure |
|---|---|---|---|
| Fan / Blower | in. w.g. | kW = 0.0001175 x CFM x ΔP / η | 0.05 to 10 in. w.g. |
| Air Compressor | PSI | kW = CFM x PSI x 0.000185 / η | 80 to 200 PSI |
| 1 PSI = 27.68 in. w.g. The compressor formula constant (0.000185) includes thermodynamic compression work derived from isothermal compression theory. | |||
Fan Affinity Law: How Airflow Changes Affect Power
Fan power scales with the cube of airflow (Fan Affinity Law 3). Cutting airflow by 20% reduces power by 49%. Doubling airflow requires 8 times the power. Variable frequency drives (VFDs) are specified and payback-analyzed using this cube relationship.
| CFM Ratio | Power Multiplier | Power Change | Example (Base: 2,000 CFM, 1.0 kW) |
|---|---|---|---|
| 50% | 0.125x | -87.5% | 0.125 kW |
| 60% | 0.216x | -78.4% | 0.216 kW |
| 70% | 0.343x | -65.7% | 0.343 kW |
| 80% | 0.512x | -48.8% | 0.512 kW |
| 90% | 0.729x | -27.1% | 0.729 kW |
| 100% (base) | 1.000x | baseline | 1.000 kW |
| 110% | 1.331x | +33.1% | 1.331 kW |
| 120% | 1.728x | +72.8% | 1.728 kW |
| 150% | 3.375x | +237.5% | 3.375 kW |
| 200% | 8.000x | +700% | 8.000 kW |
| Power ratio = (CFM ratio)^3. Valid when fan speed changes at constant system resistance. Applies directly to VFD speed reductions: 80% speed = 80% CFM = 51.2% power. | |||
Fan Efficiency by Type
The efficiency term in the formula has a larger effect on calculated kW than any other variable at fixed CFM and pressure. Fan type selection determines the achievable efficiency range at design conditions. Off-design operation reduces efficiency below peak values.
| Fan Type | Static Efficiency Range | Peak Static Efficiency | Common Applications |
|---|---|---|---|
| Propeller / Axial | 40 to 65% | ~65% | Cooling towers, low-resistance exhaust |
| Vane Axial | 65 to 80% | ~80% | Duct systems, medium-pressure supply |
| Centrifugal Forward-Curved | 55 to 70% | ~70% | Low-pressure residential HVAC, quiet operation |
| Centrifugal Backward-Curved | 70 to 85% | ~85% | Commercial AHUs, industrial exhaust |
| Centrifugal Airfoil Blade | 75 to 90% | ~90% | Premium HVAC, clean-air systems, data centers |
| Mixed Flow | 60 to 78% | ~78% | Variable flow, moderate pressure applications |
| Source: AMCA Publication 201; ASHRAE Handbook HVAC Systems and Equipment. Values are at design-point peak. Belt-drive losses (4 to 15%) and motor losses reduce system efficiency below fan shaft efficiency. | |||
Typical Static Pressures by Application
Static pressure is determined by duct layout, filter type and loading, coil resistance, and damper positions. The values below are clean-filter design targets at rated airflow.
| Application | Typical ΔP (in. w.g.) | Dominant Pressure Loss Source |
|---|---|---|
| Residential HVAC (AHU) | 0.5 to 1.0 | Filter + coil + ductwork |
| Commercial AHU | 1.0 to 3.0 | Longer duct runs, VAV boxes, MERV 13+ filters |
| Kitchen exhaust hood | 0.5 to 1.5 | Grease filters, duct length |
| Industrial process exhaust | 1.0 to 4.0 | Dust collectors, long duct runs |
| Cleanroom supply (ISO 5-6) | 2.0 to 5.0 | HEPA filtration dominates |
| Cooling tower fan | 0.05 to 0.3 | Fill resistance only, very low |
| Data center CRAC/CRAH | 0.3 to 0.8 | Raised floor, perforated tiles |
| Dirty filter conditions increase static pressure by 0.5 to 1.5 in. w.g. above clean values, reducing airflow and increasing motor load on fixed-speed fans. | ||
